Atarisoft was a brand used by Atari, Inc. to sell games for competing systems. Atari themselves typically only developed games for their own systems (Atari 8-bit computers, Atari 2600, Atari 5200, etc.) so Atarisoft titles were mostly developed by third parties. Atarisoft was only around as a brand for about two years (1983-1984), until the Atari consumer division was sold to Jack Tramiel.
It seems to me that Atari could have done better by focusing on selling their own systems and making these games exclusive, at least for a period of time. Despite the short amount of time the Atarisoft brand was used, a relatively large number of games were produced. Twenty titles were released on at least eight different systems (home computers and video game consoles other than Atari). A number of others were partially developed but cancelled before being completed after Atari was sold.
The above ad includes the earliest releases from Atarisoft for the Commodore 64, VIC-20, IBM (DOS) and TI-99/4A.