1) A taco style one is I believe one single piece of kydex, which has been folded around the spine of the knife and attached with rivets by the edge side. A pancake one is one that consists of two pieces of kydex stacked on top of each other around the knife and attached by rivets all the way around. The good thing about the pancake is that it has holes all the way around it and more places to attach para-cord or other carry devices. The good thing about the taco is that it is not as wide and doesn't take up as much space.

2) A tek-lok is a device with hinges that you close around your belt to secure it tightly after you attach the sheath to it. It has multiple holes allowing horizontal, vertical and diagonal carry, with the handle pointing in any direction. A G-clip is a simpler, slightly less bulky device, which also allows any position carry, even though it doesn't lock quite as tightly to your belt as the tek-lock.

3) From what I've heard, storing the knife in a leather sheath for a long time may cause a risk for rust if not attended, since leather is a living material. I don't believe this same sort of risk exists with a kydex sheath (given that you keep the knife in a dry place). The only real risk from use with a kydex sheath is that the blade might get some minor scratches from it.

Here's a pic of the G-clip too. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />: