The Atari 400 boots-up into "Notepad", the only built-in program. Any other programs will have to run from cassette or cartridge - this includes BASIC, or any other programming language.

Game cartridges can be inserted into the cartridge slot in front, starting instantly with no fuss.

Many games were clones of actual video arcade hits, others were original or copies of other popular (or not) computer games of the 80's.

Most of the Atari 400 circuitry is under a solid metal shield, to keep the radio emissions under the legal limit.
The two internal expansion slots are also under this cover, which can only be accessed by disassembling the entire console.
Read more about the 400's big sister, the Atari 800 for more fun and details.
Both the Atari 400 and 800 were designed by a team which included Jay Miner, who later went on to help design the Commodore Amiga 1000.