Getty Images
Andre Ward (Red) is the last American boxer to win gold. He earned his at the 2004 Summer Olympic Games.
What it takes:
Amateur boxing is not the bombastic, celebrity-studded, Pay-Per-View slugfest of Mike Tyson or the fictional Rocky Balboa. Some amateur fighters boast that theirs is a sport of skill, while professional boxing, which features longer rounds, less protection and incentives for punishing blows, is a sport that puts more value on theatrics and brute force: bloody faces, the knock-out punch.
In men’s amateur Olympic boxing, athletes from the same weight class face off in three, three-minute rounds. Women fight four two-minute rounds.
The objective is to land as many clean punches as possible and defend against the punches of an opponent. (Simply, hit the opponent, try not to get hit back.)
A knock-out punch is not awarded any more points that any other punch. In professional boxing a fighter loses points for getting knocked out. Olympic amatuer boxers do not.
How you win:

Boxers receive a point per punch landed, as long as it’s legal (not below the belt, not blocked and with the force of the body and shoulder) and three out of five judges agree. Judges use electronic keypads to award points and computers can quickly tally consensus.
Judges can deduct points for fouls: Holding, kicking, hitting below the belt, hitting with anything other than the knuckle of the glove, using offensive language, acting aggressively or disobeying the referee.
Boxers win by earning the most points, recognized by the majority of judges, overall. They can also win if their opponent gives up, becomes too injured or ill to continue, fails to make weight before the match, or goes “down”—which could mean falling down, squatting down, putting their hands on the ground—for more than 8 seconds.
What’s the lingo?

Bout: Same thing as a match, which consists of either three or four rounds.
Break: What the referee will yell if a boxer is holding or clinching the other fighter.
Clinch: When a boxer holds on to his or her opponent.
Combination: a series of simultaneous punches thrown by a boxer.
Counter-punch: a punch thrown in response to an opponent’s punch.
Jab: a straight, quick, snapping punch.
Stylist: A fighter who relies on skill and agility rather than powerful punches.
Weigh-in: A pre-fight requirement for all fighters to step on a scale and prove to officials that their weight falls within the same weight range as their opponent.
More information
USA Boxing
Boxing at London 2012
NBC Olympic Boxing Coverage