1. General: Wikipedia has a decent basic article here.
2. General: The folks over at the University of Minnesota Extension cover all the bases in their article here, with a nice diagram that illustrates how an ice dam forms and what kind of trouble it can cause.
3. General: You can see another good short article on preventing and dealing with ice dams in Popular Mechanics.
4. Technical: "Prevent Ice Dams with Air Sealing and Insulation" - an excellent, excellent article available at the Green Building Advisor website. Be sure to read this before you do anything else about your ice dam issues.
5. Technical: "Preventing Problems from Ice Dams" is a brief technical bulletin available at the website of the Canadian Asphalt Shingle Manufacturers' Association.