First of all, im sorry if the grammar from me isnt quite "correct". Im from germany and "try my best" with english, bear with me please (or help me correct it).

I also wanna credit reboot & ChaosRizer (for the "details" and the more "advanced advices") for their help

well then...lets begin.

The game has 4 classes and 8 people to choose from.

The first thing in this game is actually to choose 1 out of those 4 classes.

The Classes are:

- Officer
- Special Weapons
- Medic
- Tech

Each class has a "job" (dealing a lot damage, heal, build stuff,...) and is special made for something which i will explain futher on so you might see which class fits your playstyle best.

The Officer is a attack class with a great passiv buff (its not a constant passive buff!) for his squad members close by.
The "Leadership" buff actually gives your squad members, only if they are close by, a attack and resistence buff. Everyone close by will take less damage from enemys but also is able to do more damage. The Officer should stay close or "at front" with the squad since the squad needs this buff, helping them actually to survive better and win bigger fights.

a bit from the wiki which might helps more here:
"Each point in "Leadership" stat grants nearby marines a +3% chance of inflicting double the damage per shot/pellet for ballistic, non-explosive weapons and a +6% chance to take half damage per hit.

Each point in "Explosives Bonus" stat has the following effect on Hand Grenades and the Grenade Launcher:
Increases blast radius.
Increases damage.
For Hand Grenades, each point adds a secondary cluster detonation.

Each point also affects the following items:
Increases each Hornet Barrage's missile damage
Increases the freeze duration of Freeze Grenades
Increases the firewall duration of Incendiary Mines and firewall width (Jaeger's firewalls are approximately 33% wider than Sarge's)

The skill allows a character to place multiple ML30 Trip Mines at once with each of the 12 uses in a pack:
Sarge (and Crash who is a tech) place 2 mines at once, totaling to 24 mines in a pack.
Jaeger places 3 mines at once, totaling to 36 mines in a pack."

Mostly, the officer should use rifles or the "Vindicator" weapon since thats the weapon the officer actually gets HUGE bonus damage to.

his secondary can be everything (a second weapon, as sample) you like since he is mostly there to attack and actually helping out the squad with enemys. Sure, since they have a explosive bonus they might should go with a grenade launcher or like.

The Officer is ingame seen as the green Marine.

The "Special Weapons" (shortly SW) is a attack class, mainly. The class is able to make huge amount of damage coz their bullets are able to pierce more targets at ones so that this class is well made fighting larger swarms and bigger enemys. the SW should be armed with a autogun since the SW is the only class able to use those which are able to kill Swarms very easy.

A bit from the wiki which maybe helps more:
"The Piercing Bullets skill provides a chance for the character's bullets to pierce through enemies and continue out the other side. This skill is unique to the Special Weapons class.

Each point increases the chance a bullet will pierce its target about 20%.
Needless to say, this skill only affects conventional weapons that fire bullets. It has no effect on explosives, the Tesla Cannon, etc."

The SW has, unlike the Officer, no passive buff . If the SW is armed with a autogun, he should "shave" down such swarms very easy.

Also coz of the heavy gear the SW is using, hes also the slowest class ingame but if you put your autogun (or like) by side, choosing your secondary (hopefully its also no autogun or like), the SW runs with normal speed (if not even faster).

As secondary, the SW should take something like ammobags since his autogun needs a lot ammo(if you shot wildy around like rambo on dope...) and actually he is helping the team with it the most.

The SW is ingame seen as the Blue Marine.

The Medic is a supporter class which also has only some hitpoints ingame coz of the missing health bonus stats. The medic is one of the most important classes since its the only class ABLE to cure infestation viva heal beacon or medical gun (aka the Facehuggers which jump on you). Since the medic is pretty fast killed, the medic should never run alone around. The medic is also the only class who actually can use heal items like, as already said, heal beacon or medical guns.

A bit from the wiki which might help better here:
"The Healing Bonus skill increases the potency or amount of healing items. This skill is unique to the Medic class.

Each point grants bonuses to Heal Beacons, the Medical Gun, and the IAF Personal Healing Kit.

Beacons gain an additional charge and an increased maximum health restored.
The Medical Gun gains an increased charge capacity and an increased rate of healing.
Personal Healing Kits gain a bonus.

The Infestation Curing skill is unique to the Medic class, and reduces the amount of time required for the Medic to cure an infestation.

Each point reduces infestation duration by/about 25%."

The Medic should not run around at the front coz of his low damage and low hitpoints. hes best "placed" somewhere in the middle since from there the medic is able to heal the squad best.

A medic should be armed best with light weapons like a rifle, pistols or what ever they actually "like" to use. Its secondary should be a medical gun or heal beacon since its the only class able to use them and actually heal infestation with. Honestly, thats one bad ♥♥♥♥ move if youre the medic class and you dont even have stuff with you to heal. You will think twice about that at least if youre dieing of a facehugger...

The Medic is seen ingame as the white Marine with the red light on his bagback (which already means something like "TAKE CARE OF ME FFS!").

The Tech is a supporter class which also has the passiv buff to actually add a radar on the mini-map viva scanner. The Tech is able to show enemys around him (NOT his squad) on the minimap - the tech also says something if enemys are close by ingame (stuff like "I got a movement!" or "Careful, i see something!"). He is also the best class using Turrets coz of the enginerring bonus which means he builds them up way faster. The Tech is also for a lot missions a "must pick" since they are the only class able to actually hack consoles. A tech is a good allrounder but since in a lot missions they are "important", they actually should not begin to roam around by themselves. They should stay best with the medic since they still do good damage.

A bit from the wiki which might helps better:
"The Scanner skill increases the range of the movement scanner. This skill is unique to the Tech class. Each point increases the range of the movement scanner about 25% (but since both Techs have the same stats, it actually doesnt matters...).

The Engineering skill speeds up welding and Sentry Gun setup for all nearby teammates. This skill is unique to the Tech class.
Sentry gun setup speed is increased about 25% per level. This skill does not affect Sentry Gun dismantling speed.
Welding speed is significantly increased, although the exact times are difficult to measure.
If both Techs are taken, where the area-of-effect overlaps, the effects stack. "


Mostly armed with the PAR, the Prototype Assault rifle, they actually are at least armed to "defend themselves" and not for bigger fights. For their secondary they should best pick a turret since they can build them up very fast.

IMPORTANT INFO
A lot missions are lost if the Tech dies. If you are the Tech, try to stay with your squad and avoid friendly- and enemyfire at all costs.

The Tech is ingame seen as the brown Marine.

1. Stay together!
A single Marine is easy killed. Youre not Rambo even if you play a SW. If you think you can kill some swarms easy, that might be true but as soon as you can see you will be killed very quickly.

2. Dont panic!
A huge swarm is up your way? You are low at ammo? You found a new facehugger friend? Dont ♥♥♥♥♥♥ panic. Creating panic makes a situation worse for everyone. Keep calm and go with your team.

3. Dont be a ♥♥♥♥!
Remember: this game has all the time friendly fire. try to avoid shoothing your squad and dont do it coz its "funny". You will only ruin the fun. Also: Closing down doors behind you so that your squad cant follow counts as ♥♥♥♥ move...

4. Play as TEAM!
If the Officer roams around alone, his passiv buff will be useless.
If a SW uses pistols, he wastes his strong power with tiny guns.
If a medic has nothing to heal, thank him if you die coz youre infested.
If a Tech is not able (or duno) to hack, he should stop and change class.
Really guys, this is a team based game even with 8 people, you have a role. Try to enjoy it as team. "♥♥♥♥♥♥♥ around" is not funny.

5. Yellow = Mostly NOT good!
Mostly everything what is brown-like or yellow colored in this game is bad. tiny yellow balls which explodes and hurt you or yellow split from the enemys. Try to avoid such stuff very easy by rolling away pressing space. Remember this, its pretty usefull.

6. Listen to people who knows!
Some people maybe play this game longer and know something about this or that map, try to actually listen to them. Some people are actually long-time AS players since 2010.

7. Facehuggers? Find a medic!
ONE reason why you should not roam around only by yourself are the fact that facehuggers can kill you very very easy coz if youre not a medic, you cant do anything against them. If you got one on your head, try quickly to find your medic in hope he has something to heal.

8. (Mostly for Medics) Heal with caution!
If one of your squad members is hit ones and screaming for a medic you sure can ignor that. Why you might ask? The Medic is sure able to heal people but has only limited stuff for. he has only some beacons and the gun only has some ammo. As i can say for myself, if you play only with a few people, the Medical Gun sure heals faster (and youre actually able to move around) wile the beacon stays for around 20 seconds and is actually able to heal everyone in its radius instead of only 1 squad member. Its up to you what you think is actually better to use but i prefer in a 8 player online games the beacon, since you can heal up everyone who is hurt, which could be 1 guy or the whole squad in that case.

9. Remember ALT-Fire!
Some weapons actually are able to use the alt. fire button, which is the middle mouse button by default. A lot people actually dont know that, as sample, your rifle is able to fire 5 granades. The alt fire is mostly very limited but a very powerful addon to your weapon. You should use it. Another good sample: when using alt fire for flamethrower you shooting water, witch are quite useful on some maps when fire needs to put out or when someone is on fire

10. IM ON FIRE!!!
Its a bit like in real life that, if youre on fire, you should roll around. Its actually never said ingame but if youre on fire and actually begin to roll around, the fire extinguish way faster by itsself.

11. NOTHING HELPS against a Facehugger - ONLY a medic heal!
I had to add this since a lot people begin to panic as soon as they got a facehugger friend, punching randomly the air or fire bullets wild around them - it doesnt help in the slightest.
You are not able to "remove" your new friend by yourself. Only, and i repeat, ONLY the heal items from the medic can really remove your new friend after some time (and maybe 1-2 items let them actually JUMP off but doesnt actually "kill them"...). just get heals or stay in a beacon. As for the medic, dont spam/waste all your beacons on ONE infested member. Use one and let them cure. If he needs another, use one more BUT dont begin to panic and spam like 5 beacons around...

MidGuard asked:
"... If you want to expand it, I think a section on position would really help. Just a bit about friendly fire, not running around in front of teammates, firing lines, all that. Would really help since many people love to panic and strafe like mad in front of the entire squard during a tense firefight lol. "

Answer:
The "position" Is something which is really important. As MidGuard already said, a lot people just keep firing and dont remember the game actually has friendly fire.
So how should the position of a team look like?
At front there should be the Officer coz hes the best class with his buff and mostly "close range" weapons. next to him the SW followed by the Medic and the Tech.
Running together as squad that everyone is able to do damage is best if the Officer walks always a bit by side and not in the middle of the hallway so that the SW can shoot straight into the enemy lines followed on his left and right shoulder from the Medic and Tech.

Some other good advices (by ChaosRizer)
Those advices are also pretty good for advanced players, just said.

Useful Gadgets to pack up when you´re uncertain:
-Elec Armor helps in removing parasite(may take awhile and stun nearby attacker(s)
-Tesla Trap(limited charges but mini-stuns multiple bugs on being shock+slow?YES PLS)
-Freeze Nades(freeze em for bit of time to give you some breather)
-Combat Flares(unlike stock campaign previous version of game,this new revamped version has some dark places where Flares can be advantageous)
-Flame Mines(OFFICER):Firewall of doom.Nuff said.
-Mines(Benefits OFFICER more in which OFFICER can toss more Explosive according to his Explosive Rating) Laser Mines to show em "the moderns of ambush kablooey".
-Adrenaline 10(16,Bastille specific) seconds of "Bullet/Lag Time" just what the team can use in a pinch,occasionally.

Fire your weapon in semi auto/short bursts(unless in horde/clear firing line) will ensure that you dont need to rely on ammo restore more.Flamers benefit mostly from this as shooting in short burst creates flame "puffs".

When in doubt,you can use melee to smack a friggin hand-mook,smash obstacles,crush doors(not so recommended tactic on crushin doors)....

You can roll to tranverse certain obstacles in game,as well as rolling away from LoF.

Beneficial Secondary(opinions vary but this is my personal "best" one in my mind)
-Tesla Rifle(does no FF and chain shock which results in mass stun+slow.)
-Sentry/Freeze Turret(The latter doesnt deal damage but chain freezes horde.)
-Ammo crates(moar dakka hur dur)
-Flamenwerfer(duh)

Hold Shift while moving to walk,or to crouch when being still.

SW Autogun,Minigun and AA42 slows their movement.You can swap to secondary to return to normal marine speed.Either pack ammo crates or another secondary to utilize this.

Officer also gains bonus towards Explosives such as Laser Mines,Grenade Launcher or Grenade Sentry.Caution is advised for using Grenade Turret/Launcher as it may kill allies.

Alien Swarm
A game original back from 2010.
Here the original link to it for people who actually never played or saw the original AS on steam, sure, since the new version is out, it makes no sense to play this anymore

Its also free but is very limited.
http://steamproxy.net/steamstore/app/630/

I hope i helped you to understand the classes better and gave you some usefull advices on the way.

If you like my guide please give it a like and vote for it. I thank you in advance and im also glad if i could help you. We see us ingame.