Tom's Hardware has a good article, it's a couple years old but covers the basics> http://www.tomshardware.com/reviews/pc-memory,1698-2.html
Damaged is another story, I would not call it damaged necessarily more a matter of a failed component or one that was not 100% in the first place and deteriorated with use. Damaged ram generally comes from over volting or sometimes voltage spikes and usually effects the entire stick.
