They got rid of the card slot, the device was smaller, and it had an inbuilt game (Alex the Kidd in Miracle World in my case, although it seems some had Sonic instead)

There even was am even smaller version called the Master System III which I think only was released in Brazil.

The original Mark III was what would be the Master System in the west. The Mark 1 in this case was the Sega SG-1000 which had a very limited release outside of Japan (and came out the same day as the Famicom, so always had an uphill battle). The mark 2 was an iteration om that. As was the Mark 3 and the later Mega Drive actually.