Valve Anti-Cheat is a software developed by Valve, Inc. as a method to detect and ban cheaters.
How Does It Work?
VAC, if implemented into the game, will silenty check over programs running on the computer and memory as well. When the system (and if it) finds something odd running, it will be looked over by Valve themselves, OR, if matched up to a cheat program that Valve has already detected beforehand, a VAC ban will most likely be issued.
When Was It Developed?
The earliest version of VAC was created in 2001.
All VAC bans are permanent.