Torque converter should be on howstuffworks, and will explain it way better than I ever will. As for checking it, you really can't, that's one of the things I'd take to the shop unless you've got a couple friends who are good with cars, or atleast very patient. I don't know much about them however, cause I don't have an automatic.
Kevin (President TBEA)
"WAAAAAAARRRRRIORS, come out to plaaaaay."