| In the sport of Boxing, a low blow is often thought of as a punch that lands directly to the groin sending the receiving fighter to the ground in extreme pain.
A low blow is defined as any punch that lands on or below the belt to the referee's discretion. So a punch to the thigh is also a low blow. Usually at either the beginning of a fight, or before it begins the ref will tell the fighters where the line is between a fair punch and a low blow. This is mainly to deal with fighters who feel the need to wear their shorts like an old man in order to discourage body blows.
A low blow is, of course, a foul in Boxing. Usually a referee will give two or three warnings to a fighter who has thrown low blows. After that, the referee will start taking away points. This is completely at the discretion of the ref. If a boxer blatantly throws their first low blow the ref may deduct a point right away. Point deductions in a close fight can make a big difference. After the ref starts deducting points the fighter will usually be disqualified if he throws several more low blows.
When a fighter recieves a low blow the ref will determine how badly the fighter has been injured, or how much pain he is in. The injured fighter can be given up to five minutes to recover from the low blow but the amount of time given is up to the ref. Sometimes refs will just say, "Come on, let's go" and other times they'll give the full five minutes. If the fighter decides not to continue after this he will lose the fight.
Low blows occur fairly often in modern boxing. Usually they are caused when a fighter is trying to deliver a legal body blow. Differences in stance, height, fighting style, etc. can sometimes lead to fights with many accidental low blows. | 
