The normal nitro is used when pressing the nitro button only once.


PROS:
Efficient
Longer duration

CONS:
Speed easily lost after steering
Jank control over vehicle

The normal nitro is for straight roads. It's duration is pretty long compared to the other nitro types.
Normal nitro may be weak but it's really efficient if you're driving in a straight line.

Burst nitro is activated by tapping the nitro button while you are using normal nitro.


PROS:
Faster acceleration
Lower chance at getting knocked down

CONS:
Bad vehicle control
Short duration

Burst nitro is often used while driving up a ramp to keep maximum speed. The best way to use it though is to defend yourself and accelerate when gotten slowed down.

Perfect Nitro is performed like Burst Nitro but only when pressing the Nitro Button while your Nitro is in the blue area.


PROS:
Perfect control over the vehicle
Decent duration (between normal and burst nitro)
Always on maximum speed

CONS:
(Literally none)

The Perfect Nitro is the "allrounder".

And finally the Shockwave Nitro.


A full Nitro Meter will blink with a purple'Ish colour indicating that a Shockwave is ready.
You perform a Shockwave by rapidly pressing the Nitro button twice.


PROS:
Easily knock down opponent
Fastest acceleration

CONS:
very short duration
very bad control of the vehicle
cant be activated any time

The Shockwave is very powerful but only lasts for a few seconds. You can use it like Burst Nitro but it should only be used if you got slowed down a lot or try to knock an opponent down.

The Nitro Upgrade usually upgrades the Duration of your Nitro and increases Nitro speed.
Edit 8/20/2023: However there are a few cars where the Nitro upgrade only affects the duration.