Rules

The following describes some rules commonly found in MMA competition in most MMA organizations.
Ways to victory
Knock Out (KO): as soon as a fighter becomes unconscious due to strikes, his opponent is declared the winner (because MMA rules allow ground fighting, the fight is stopped to prevent further injury to the unconcious fighter.)
Submission: a fighter may admit defeat during a match by:
tapping three times on his opponent's body;
tapping three times on the mat or floor;
verbal announcement.

Technical Knockout (TKO)
Referee Stoppage: the referee may stop a match in progress if:
a fighter becomes dominant to the point where the opponent is unable to intelligently defend himself from attacks (this may occur as quickly as a few seconds);
a fighter appears to be unconscious from a grappling hold.
a fighter appears to have developed significant injuries (such as a broken bone) in the referee's view.

Doctor Stoppage: the referee will call for a time out if a fighter's ability to continue is in question as a result of apparent injuries (such as a large cut). The ring doctor will inspect the fighter and stop the match if the fighter is deemed unable to continue safely, rendering the opponent the winner. However, if the match is stopped as a result of an injury from illegal actions by the opponent, either a disqualification or no contest will be issued instead.
Corner stoppage: a fighter's cornermen may announce defeat on the fighter's behalf by throwing in the towel during the match in progress or between rounds.

Decision: if the match goes the distance, then the outcome of the bout is determined by three judges. The judging critera are organization-specific.

Forfeit: a fighter or his representative may forfeit a match prior to the beginning of the match, thereby losing the match.

Disqualification: a "warning" will be given when a fighter commits a foul or illegal action or does not follow the referee's instruction. Three warnings will result in a disqualification. Moreover, if a fighter is injured and unable to continue due to a deliberate illegal technique from his opponent, the opponent will be disqualified.

No Contest: in the event that both fighters commit a violation of the rules, or a fighter is unable to continue due to an injury from an accidental illegal technique, the match will be declared a "No Contest".

Weight categories
Although each organization divides its fighters into weight classes, the details are organization-specific.

Fouls
No head-butting, eye gouging, hair pulling, biting or fish hooking (pulling at the cheek with a finger).
No attacking the groin
No strikes to the back of the head, spinal area and kidneys.
No strikes to, or grabs of the trachea
No small joint manipulation (control of four or more fingers/toes is necessary).
No intentionally throwing your opponent out of the ring.
No running out of the ring.
No purposely holding the ring ropes or octagon fence.

Rules variations
Each organization determines its own rules (in accordance with government regulation). Below are some of the significant differences in the rules of the popular MMA organizations.

Ultimate Fighting Championship
Allows elbow strikes except downward elbow strikes with the point of the elbow.
Prohibits spiking a fighter onto his head during takedown or slam.
Prohibits stomps, soccer kicks and knees to the head of a grounded opponent (more than feet touching ground). A fighter on the ground can kick upwards at their opponent's head only if their opponent is standing.
Prohibits shoes, shirts and pants.
Uses three 5-minute rounds. Championship bouts are five 5-minute rounds.
No longer uses a tournament format.
Has five weight classes: Heavyweight (<265 lbs), Light Heavyweight (<205 lbs), Middleweight (<185 lbs), and Welterweight (<170 lbs) a Lightweight (<155 lbs) class.
Tests fighters for steroids and other illegal substances in championship bouts.

Pride Fighting Championships
Uses a 10-minute first round with 5-minute second and third rounds.
Prohibits elbow strikes to the head.
Allows stomps, soccer kicks and knees to the head of a grounded opponent.
Uses tournament format to award Grand Prix champions.
Has two weight classes: Heavyweight (no limit), and Middleweight (<92 kg).
"Bushido" event series consists of lightweight (<73 kg) and light-middleweight (<83 kg) fighters.

K-1 HERO's
Uses two 5-minute rounds, with an extra round option should the judges be unable to determine a clear winner of the fight.
Prohibits elbow strikes to the head.
Weight classes are currently being established. Lightweight is under 70 kg..