More than you EVER wanted to know about gears, gearing and why.
Ken Kifer's Bicycle Pages.
The late Ken Kifer was run over by a car. These pages were written by him before the fatal accident and are retained in his memory. Ken was a good friend to all cyclists and he is now going over the top of the worst climbs in the big ring.
http://www.kenkifer.com/bikepages/touring/gears.htm