The 800XL came with 64k of RAM. Most users wondered what we would ever do with that much memory. An external 5.5 inch floppy disk or cassette deck drive was optional.
The entire computer was built into the keyboard. Atari computers generally used an external converter that reproduced both video and audio through a television.
Atari computers rapidly lost market share in the 1980s due to Atari's preoccupation with video games and game consoles so that when Atari employees Steve Jobs and Steve Wozniak developed the first Apple computer, they had to start their own company to produce it.
Atari was the Home Computer to have until the Apple came out.
As was standard for the era of Atari's golden age, their computers only had rudimentary operating systems, so most operating instructions were written into each software application.
Their primary competitor, Commodore, entered the market late, with a Home Computer that wasn't good for much besides games and greeting cards, even though Atari was still better at those tasks due to better color and sound support. For a time, more Commodores were sold than Ataris due to better marketing. Other players in that era included the Timex/Sinclair and the TI 99/A.
Atari eventually developed IBM-compatible PCs in the late 1980s, but it was too little, and far too late.