VAC stands for Valve Anti-Cheat, an automated system designed to detect cheats installed on users' computers. If a user connects to a VAC-Secured server from a computer with identifiable cheats installed, the VAC system will ban the user from playing that game on VAC-Secured servers in the future.

The VAC system reliably detects cheats using their cheat signatures. Any third-party modifications to a game designed to give one player an advantage over another is classified as a cheat or hack and will trigger a VAC ban. This includes modifications to a game's core executable files and dynamic link libraries.

While server admins may choose to ban specific players, server admins cannot VAC ban players.
The following will not trigger a VAC ban:

Using chat programs like X-Fire or Overwolf
System hardware configurations
Updated system drivers, such as video card drivers