The Atari 5200 SuperSystem, commonly known as the Atari 5200, is a video game console that was introduced in 1982 by Atari Inc. as a higher end complementary console for the popular Atari 2600.
The 5200 was created to compete with the Intellivision, but wound up more directly competing with the ColecoVision shortly after its release.
The 5200 was based on Atari Inc.'s existing 400/800 computers and the internal hardware was almost identical, although software was not directly compatible between the two systems. The 5200's controllers have an analog joystick and a numeric keypad along with start, pause and reset buttons. The 360-degree non-centering joystick was touted as offering more control than the eight-way joystick controller offered with the Atari 2600.
The 1983 revision of the Atari 5200 has two controller ports instead of four, and a change back to the more conventional separate power supply and standard non-autoswitching RF switch. It also has changes in the cartridge port address lines to allow for the Atari 2600 adapter released that year. While the adapter was only made to work on the two-port version, modifications can be made to the four-port to make it line-compatible. In fact, towards the end of the four-port model's production run, there were a limited number of consoles produced which included these modifications. These consoles can be identified by an asterisk in their serial number.
More details on Wikipedia.
What's it worth? Take a look at this Atari 5200 price guide: sold listings for a value indication.