Valve Anti-Cheat. Basically, it's a program that is like a virus scanner only for hacks, if they detect hacks on server joins and whatnot, you will be banned within a few days, hours, or weeks.
Players who have been banned always claim they didn't cheat, or their brother or friend put the hacks on, basically trying to get it out. There have only been less than 10 people actually unbanned from VAC before. Anyone who is banned will only be banned from VAC secure servers, but can still play on unsecured ones. Every game that uses VAC will be inaccessible (you won't be able to play on VAC secure servers).
It's often criticized as a way to make money from Valve to get a new account and games. Accounts that have been banned payed up to thousands on games before, but over 1 hack they loose all.
Many times VAC isn't updated to the most recent hacks, private hacks still exist, and are harder to detect.
VAC has certainly busted many cheaters, but some still get by.
Noob: OMG! Me is VAC banned! WTF! vavle u just want my monies! I didtn hack!
Me: God shut up. We all know you hacked, headshots 24/7 isn't easy, but you managed to do it. Go away.