Alt Gr is a rather hardman version of the Alt key.
Alt turns into Alt Gr like this:
Alt: I'm having more beer.
Alt: I want more beer!!
Windows: I think you've had enough already.
Alt: ALT GR!!!!!
*Linux presses Alt Gr key, instantly giving Alt Gr more beer.*
Linux: On the house!
*Alt Gr smashes Windows, Linux, and the other regulars in the face*
Alt Gr: ALT GRRRR!!!