Derailleurs, fortunately have been fairly standardized. Simple specs like these can help you determine what derailleur is best for your bike. Here's a brief overview of some of these:
- Maximum capacity: The maximum number of teeth between the largest and smallest rings. eg. 48T-22T = 26T.
- Top gear teeth: The range available for the largest ring. Any size ring in this range will work. Even some rings outside this range may work, but not to their best potential.
- Cable routing: Derailleurs come in multiple pull types:
- Top Swing: The clamp is below the cage and the cage pivots above the clamp. The cable attaches from above.
- Bottom Swing: The cage is below the clamp and swings from below the clamp. The cable attaches from below.
- Dual Pull: Can pull from either direction.
- Chain Stay Angle: The chainstay angle is the angle between the chain stay and the seat tube. The derailleur must fit this angle or risk the cage hitting the chain stay.
Of course, Sheldon Brown has some good information, but if you really want to dive into all those numbers, have a chat with a mechanic at your LBS. I didn't know anything about these angles until my mechanic pointed out that my seat tube angle changed near the BB and meant I should have a bottom swing instead of the top swing I bought. It works, but doesn't provide the best shifting possible.