Along with it's bigger sister the 800, this machine was part of Atari's foray into the 8-bit home computer market after the success of the VCS. Released in 1979, the 400 is essentially a cut-down 800 - not exactly 'half' an 800 because it's possible to build up the 400 to be just as powerful. The only real difference between the 2 is less expansion, no composite video out and a membrane keyboard.
The 400 was also aimed more at kids with its funky styling (never copied by anyone else, fact fans) and spill-proof membrane keyboard that's just as fiddly to use as the Sinclair ZX80/81. At least you got audible feedback with this one! Safety features like a power switch under the cartridge flap ensured kids and grown-up kids didn't fry the machine's internals by trying to insert or remove a cart with the power on :)
I/O was restricted since the original 16K of RAM didn't have enough space to allow disk commands, so the 400 was released with its own tape deck - the 410. Later on when 3rd party upgrading got pretty snazzy it was possible to have 64K of RAM (same as the 800) and use a floppy drive. Other upgrades were add-on sound/video outputs and at least one 'real' keyboard that you can see here. Nice to type on too.
Despite being 6502 based like a lot of the other machines that were being developed (Acorn Atom, Apple II etc), the 400 used separate co-processors to take some of the load off the CPU. ANTIC and CTIA (later GTIA) did the screen and player objects, POKEY did the sound, keyboard and serial I/O. If all this is sounding familiar bear in mind one of the designers of (I think) ANTIC was Jay Miner, who went on to design some of the custom chippery in the Amiga 1000
Another useful feature was that 90% of the machine was not country specific - the CPU, ANTIC and CTIA/GTIA were mounted on a separate board (as was RAM), so to change countries you just changed the daughterboard - it's the one in the pix with the 3 warranty stickers on. Different power supply too, natch.