Composite are a softer material and wear down faster, but they tend to stop smoother due to the soft material. They also have less black dust build-up due to the organic material.
Semi-metalic have metal inpregnated into a carbon material. They last longer and stop great. The carbon material causes the excessive dust build-up.
Ceramic last the longest. If you have a problem with brakes wearing down to fast, these are the way to go. The downside is cost. These brakes cost about 2-3 times more the semi-metalics.
I've heard about a titanium pads for extreme duty and racing. These make the ceramics look cheap. Other than that I've not heard any more info of them.