TV wise what are the real differences in an LCD, LED, and plasma TV?

I was with my best bud he was looking over the big screens and home theater sound while I shopped for ink and such at the electronic store. He said he loved the TV he was looking at because it was LCD. Looking at the picture I could not tell the difference from the LCD or the LED; he liked the LCD because they were cheaper. The plasma TVs were no that sharp (picture wise) to me. I asked him to explain what the difference was between LCD, LED, and plasma. I know LED the screen is light admitting diodes, LCD is _light crystal display, I have no ideal what a plasma screen is made of, but anyone know what seperates them and can put it in a nutshell?