Valve has decided that Counter-Strike: Global Offensive players are mature enough to police themselves, so it launched the Overwatch where they are able to do so.
The Overwatch system chooses "reputable" members of the game’s community automatically and nominate them as "investigators." Other players in the game are then able to click a special "Overwatch" button to alert investigators to in-game cases.
When the Overwatch button is pressed, the investigators assigned to the case will be able to watch eight rounds’ worth of game replay (about 10 minutes) before deciding whether the suspect has engaged in behavior deemed Majorly Disruptive (cheating), Minorly Disruptive (griefing), or there is Insufficient Evidence. A verdict is then reached according to the judgment of the "overwhelming majority" of investigators assigned to the case.
As an extra precaution, Overwatch assigns old cases that have already been decided in order to asses their objectiveness and competence.