What is "GEM"?

I am new to this and having problems with my rear passenger power window on a 2000 mustang. I have been reading some stuff that refers to a GEM however I do not know what that stands for. Could someone explain it to me.
Thanks.
 
Thanks. Would replacing the GEM be an expensive undertaking? It sounds expensive. I have one more thing to check (the wiring at the door jam) and then it may become a manual window. Remove the motor and put it up and down by hand. It is a convertible so it looks stupid with one window up when the top is down.