There are basically two main types of martial arts, the kind that inject new energy into a fight via you throwing punches at somebody, and the kind that is 'redirectional' or in other words someone throws a punch at you and you grab their arm and throw them striaght into the floor and use their own energy to work against them. I'm a big fan of redirectional martial arts.

Martial arts where you are expending your own energy are things like Karate and Tae Kwon Do. Martial arts where you are mostly redirecting are things like Akido, Judo, and Jiu Jitzu.

Something else to consider is the fact that you often times have the same martial art defined in two different ways depending on how you plan on using it. Judo and Jiu Jitzu for example are very similar, but Judo is more for sport while Jiu Jitzu is more for fighting. You do some dumb things in Judo that you would never do in a real-world fight, like turning your back to the attacker in some moves.

Finally, martial arts do things in different ways. Some are focused mostly on arm/hand movement, while others involve a great deal of kicking (e.g. Tae Kwon Do) and still others are very heavily into wrestling/grapling and focus on you trying to take an attacker down on all fours and pinning him.

It's been a long time since I was involved in any of this and unfortunately that is about all I can remember right now. Do some research and find one that you like that meets your needs. Good luck.