design - Beginner information related to constant current dummy loads

I am beginning an electronics design project to build a constant current load to drive testings for batteries, bench power supplies etc. I have been hard trying to locate some black-on-white information over the Net to get the basics: what is the main operation principles of PWM controlled constant current dummy loads and, what do the particular parts like operational amplifiers of the circuit do. I think, I have been trying to find these basics with wrong search string but can't help myself anymore to invent some other words than "dummy load basics" or so. I hope you guys are able to help me in finding the answers.