Yeah, I know. To keep it simple, there are basically two families of systems. 2-hole "mountain bike" systems, and 3 hole "road bike" systems.

The 2-hole cleats are small (like 1"x1/2")and are secured by 2-bolts, hence the name. SPDs fall into this category. The cleats are designed to be recessed in the bottom of a mountain bike shoe so you can walk around. Most pedals with this system also have dual sided clip in, so you don't have to make sure the pedal is flipped a certain way. Egg beaters have 4 sided entry.

The 3-hole cleats are much larger (2"x2"x2" triangle). These bolt directly to the outside of a road shoe, which has no tread. The cleats are exposed and walking is harder. The pedals are only one sided entry, although they are weighted such that they always hang in one orientation, so it's still pretty easy to find the right side without looking.