"From The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips. Over the following decade several versions of the same basic design were released, including the original Atari 400 and 800 and their successors, the XL and XE series of computers. Overall, the Atari 8-bit computer line was a commercial success, selling two million units through its major production run between late 1979 and mid-1985.
Although the computers had varying specifications and a few different cartridge formats, there was a high amount of interoperability between them. Later models, in particular, are capable of playing almost all Atari 8-bit family games."
Source: Wikipedia, "Atari_8-bit_family", available under the CC-BY-SA License.