Martial arts which fall into the combat sport category focus on defeating another skilled opponent. You are basically training for a duel against a highly-trained adversary. There is a high emphasis on strategy and developing fight I.Q., which is the ability to adapt to your opponent's reactions in the heat of the moment. The most lethal techniques (i.e. attacks to the groin, throat, and eyes) are banned for safety reasons. Most of the time, you will fight in a controlled environment such as a ring, which has mats made out of canvas or other soft materials to prevent injury. Combat sports include boxing, kickboxing, wrestling, MMA, judo, Brazilian jiu-jitsu, and capoiera, in which the fight is typically disguised as a dance.
The chief benefit of combat sports is that they provide a safe environment in which you can practice your techniques against a real person. Even in full-contact bouts, there are referees and doctors to minimize the damage you will take in the match. Combat sports also include exercises to improve your timing, accuracy, distance management and other valuable skills which will help you if you are attacked by a real assailant. Not to mention that they are a fun way to get to know other martial artists.
The main drawbrack of combat sports is that they only prepare you for a single foe with a code of honor. Since the most lethal self-defense techniques are "against the rules", they are barely covered, if they are taught at all. Due to this, much of what you learn won't work against multiple aggressors who are trying to kidnap, rape, or kill you. Additionally, most combat sports have weight classes, gender and age divisions to keep the fights fair. This is not realistic: most assailants are big, strong, young male hoods who could care less about fighting fair. A technique which works on someone your size may not necessarily work on them. For example, if you are a 100-lbs. girl, that right hook to the jaw might work against other 100-lbs. girls. But if you use that against a 250-lbs. musclehead, you'll probably wind up with a broken hand and the worse beatdown of your life before the inevitable occurs. Also, most combat sports don't teach you defenses against armed opponents. The ones that do (such as fencing and kendo) don't teach defenses against modern weapons. The few combat sport instructors who cover modern weapons' defenses tend to teach ones which have never been battle-tested. These techniques are really just guesswork on their part and not anything that you would want to bet your life on.
You should study a combat sport if you already have a firm grasp of modern self-defense and want to see how you measure up against other skilled martial artists. If you want a fun hobby that allows you to meet cool people, combat sports are also for you. If you want to compete professionally, that's fine, too. Just make sure that you start as young as possible (I suggest by your early twenties at the latest). It would be wise to have a practical college degree to fall back on as well.
Next week, I will cover the final type of martial art: traditional martial arts.