How do I report a suspected cheater in a VAC secure game?
While it is not necessary to report cheaters playing on VAC-Secured servers - the VAC system will detect and ban these individuals - players who are suspected of cheating may be reported via the Steam Community.

Note:
VAC does not ban based on reports. Such data is only used for tracking and statistics purposes.

To report a cheater:
Go to the user's Steam Community profile
Click on the large More button in the upper-right corner of the user's profile
Click on the "Report Violation" button in this dropdown menu
Select "Suspected Cheater", add a description (include the game and the name of the cheat if known), and hit "Submit"

To reach a user's Steam Community profile while in-game:
Press shift+tab (by default) to bring up the Steam Overlay
In the Friends panel on the left, click "View Players"
Find the offending player under the appropriate tab and click "Profile"

To report a cheater in Counter-Strike: Global Offensive:
Reporting cheaters in Counter-Strike: Global Offensive will help identify users that should be viewed by Overwatch.
While holding the scoreboard key (default: tab), press your alternate fire/zoom key (default: right mouse button) to show a mouse cursor.
Right-click the name on the scoreboard, and click "Report".
A new dialog box will appear. Click the appropriate checkboxes and click 'Submit'.

More information on the Valve Anti-Cheat System (VAC):
https://support.steampowered.com/kb_article.php?ref=7849-RADZ-6869