VAC is a Steam anti-cheat (detects illegal software) system which automatically detects unauthorised software.
If a player is banned by VAC, he/she will receive the following penalties.
1) Cannot purchase any VAC applied Steam games with this account.
2) Cannot play VAC applied games.
3) Cannot appeal. If banned by mistake, the account will be unbanned after a certain period of time.
4) Cannot receive VAC applied game as gifts.
5) Cannot merge items to another account.
6) Cannot upload content on Steam community for VAC applied games.