So call Cannondale's tech dept. Locate a dealer on their website. Good options there.

http://www.cannondale.com/company/index.html

Also, try www.parktool.com. Look for service help and you'll get a diagram of a bike. It will help you identify parts.

The cassette is the set of gears attached to the back wheel. The derailleur shifts the chain across these gears. The hanger is the piece the derailleur attaches to on the frame. The dropouts are the parts to which the wheel skewer attaches.