Original: Among the criticisms raised towards the initial version of the Atari 5200 is that it is fully incompatible with the software library of the Atari 2600. However, a revised 2 port model of the 5200 along with an Atari 2600 game adapter was released in 1983, allowing it to play all Atari 2600 games.
Simplified: It was made by Atari Inc. and was supposed to replace the popular Atari 2600.