Valve has decided that players in the Counter-Strike: Global Offensive community should be able to police themselves. With that in mind, the company has launched a beta of a new system it has called Overwatch. The system will choose reputable members of the game's community to serve as investigators. When playing the game they'll have a special "Overwatch button" that will alert them that a case is pending to evaluate.
Assuming that they decided to hit that button, they'll watch eight rounds' worth of replayed gameplay (about 10 minutes) and be asked to judge whether "The Suspect" was engaged in behavior deemed Majorly Disruptive (cheating), Minorly Disruptive (griefing), or there is Insufficient Evidence. Investigators can also postpone their verdict for a later time. If all investigators reviewing the case agree, a ban will be issued - with the severity of the duration dependent on seriousness of the offense. Obviously, if all investigators agree that there's insufficient evidence, the case will be thrown out. While Overwatch is in beta, the system will issue fewer cases and the verdicts will be reviewed before coming into effect.
Potential investigators chosen by Valve will be given cases that have already been decided on in order to test if they have the right judgment and understanding to handle the position.
You can find out more about Overwatch via this FAQ.