The ST straddled the two worlds, offering separate color and monochrome screens, auto-detected by the computer's display circuitry. At the time, Macs were solely monochrome, and Amigas were solely color. The computer primarily competed with the Macintosh and the Amiga in most markets. It ran Digital Research's GEM (Graphical Environment Manager) on Atari's proprietary TOS (The Operating System). 'ST' stands for sixteen/thirty-two, in reference to the 16-bit external bus and 32-bit internals of the 68000 chip.
It was announced in January of 1985, and released in June of that year. The Atari ST was a microcomputer developed by Atari Corporation in the mid 1980's.
