Need Info Re Bike Buying

[Derf]

A friend just announced that she wants a bike. She is a complete noobie. Does anyone know of an article online that explains the differences in types of bikes as well as materials, etc? Thanks!

[KnottedYet]

Sheldon Brown's got some great beginner articles.

http://sheldonbrown.com/beginners/index.html

and Wikipedia is full of goodies!
http://en.wikipedia.org/wiki/Bicycles

[Kimmyt]

And then there's always: http://www.teamestrogen.com/beginnerCyclist.asp