At this point amateur MMA is still new and the rules can vary from state to state.
Amateur rules usually involve more pads (i.e.- shin pads and thinker gloves), shorter rounds, restricted striking (i.e.- no knees to the head at all, no head shots while on the ground, ect) and do not allow the fighters to have any pro fights. And most importantly, amateur fighters do not get paid.
Up until recently almost all MMA was “pro”, because there was no amateur system. So guys would be stepping in the ring for the first time, fighting as pro’s.
Amateur MMA has been around about as long as pro has. My first fight was in 99, and it was amateur.
As far as extra padding, in 5 amateur fights, I have never worn shin pads or thicker gloves. My one pro fight was Shooto B class in Guam, and that had thicker gloves than any of my amateur fights(Shooto gloves versus Everlast/Chuck Norris/Harbinger-type fingerless gloves). First ammy fight in FL - no head shots on the ground. Two ammy fights in IN - open hand to the head on the ground. Two ammy fights in VA - closed fist standing or ground, kick and knee permitted standing(basically same as UFC minus elbows).