What type of bullets do semi automatics take?

In most cases, center fire cartridges or shot-shells. It depends on what type of firearm you are using(handguns, shotguns, rifles), but the spectrum ranges anywhere from .17(small) to .95(extremely large) for rifles and handguns. Shotguns can go anywhere from a 32-gauge to a very large 8-gauge. If you have further questions, go to Wikipedia for more precise information.