VAC works by scanning your system while you play any VAC supported game (TF2, L4D1+2, etc). I don't know what exactly it scans for, and in fact, that is probably not public knowledge at any rate.
Bans are unappeal-able, and final.
When it kicks in, you are unable to play online multiplayer with other players who are in goodon VAC standingsecured servers.
