I checked the FAQ and I hope this hasn't been asked before... but how did medieval knighthood really work?

I was reading a discussion that broke out on /r/asoiaf the other day about how accurate the knights of Game of Thrones are, and one user described them as essentially the medieval version of a modern protection racket (more brutal and thuggish but protecting peasants/serfs in exchange for what they produced) while another described them more honourably, like a mixture of policeman and elite soldier.

Reading that exchange made me realize how little I know about the actual historical knights of the past. Obviously the classical fictional stereotype of "the noble knight" is somewhere between Prince Charming and Superman... but while I don't think they were all thuggish murdermachines, I doubt they were all that valiant either.

So I ask you guys, how brutal and/or chivalrous were the actual knights of medieval history? What were their usual duties, both at peace and at war? Did it vary widely based on birth or merit?

thanks

^(If it helps to specify a region/time period then I'm most curious about England/France sort of area. I guess a period when they were at their most relevant and important.. sorry, if I knew more about them I'd be more specific)