A low blow is any punch that falls below the beltline.1 In other words, if you punch someone in the gonads, the privates, the family jewels, the place where the sun don't shine, etc... you have just given a low blow.
In boxing, you get three hits before a point is deducted. It is an illegal hit and when it happens, your opponent has up to 5 minutes to get himself ready to fight again (at least in professional boxing). This is at the discretion of the referree who makes the decision based on what he thinks is the severity of the blow.
It is also up to the ref when a point is deducted. If it is obviously intentional, he may not wait for three before you issuing the penalty. In a normal match though, the first hit to the groin is generally considered an accident. The second will receive a warning and the third results in the point loss.
It may seem all fun and good to nail someone where it hurts and risk the penalty, but if it is late in the fight, 5 minutes of rest may be just what your opponent needs to break your rhythm, recuperate and come back at you even stronger - especially if he was out of breathe. He may even welcome the hit.
Check out the low blow Hatton gave Tszyu in 2005.