I went looking for a shower door recently, and was confused about all the types that were available, such as what a by-pass door was. I found a pretty good site that had all of the door definitions on it http://www.americanshowerandtubdoor.com/definitions.cfm. After I figured out what the right door was I could picture it, but I didn't want to just buy the cheapest quality stuff at the local big box. I actually ordered one from this store, and installed it.
There are several types of shower doors as you have found. A bypass door is a sliding door where the two panels slide past eachother. There are different grades of these but basically they are pretty much all the same. One thing to look for is quality of the hardware, weather the hardware goes through the glass instead of gripping the edge of the glass and if the glass is 1/4" thick as opposed to 3/16. American shower door is good, so is Contractors wardrobe and a few others.