The Atari 130 ST (ST means sixteen/thirty-two :16 bit data bus & 32 bit address bus) was shown at the Winter Las Vegas Consumer Electronics Show in 1985. It was the great surprise of the show and was presented only six months after Jack Tramiel bought Atari, Inc. (He later named it Atari Corp).
The ST series was rushed onto the market as Atari had originally contracted Amiga Corp. to manufacture a 16-Bit home computer, but legal issues caused the dissolution of that contract resulting in Commodore Computer releasing the Amiga and Atari creating the ST using 'Off The Shelf' electronics to release a computer to coincide with Commodore's release of the Amiga.
It was also designed to be a "Macintosh Killer", and was jokingly called the "Jackintosh". It offered lots of features never found on home computers before: 16 bit microprocessor, lots of interfaces, standard MIDI interface, high graphic resolution, lot of colors, enhanced GUI (Graphic User Interface). It was much more powerful and expandable than the Macintosh (MIDI, graphics, colors, and the like) and competitors weren't very numerous (Commodore 128, PC AT, & compatibles, and the old CP/M machines). That explains why more than 6 million of Atari ST's were sold around the world.
The Atari 130 ST was a prototype for the press demonstration and was never released because of its small memory size : the complete operating system couldn't fit in it. It was replaced by the Atari 260 ST and the Atari 520 ST. Despite its name, the Atari 260 ST was shipped with 512 KB of RAM, because the 192 KB operating system left less than 64 KB of free RAM.
The Operating System was TOS (Tramiel Operating System, later renamed just "The Operating System") which was in fact no more than the old CP/M 68K. Notice that to make easy the transition from the PC to ST, all the high level calls of the TOS (called GEMDOS) were compatible with the DOS calls of the PC (int 21h).
All of the ST family systems used the GEM, which stands for Graphic Environment Manager, GUI (from Digital Research) which has a striking resemblance to the Macintosh GUI (windows, dialog boxes, desktop, desktop accessories, & bitmap fonts) even if it is a bit less friendly than the Apple one.
The 260ST Operating System (TOS & GEM) was supplied on disquette with earlier models, then into 6 x 32 KB ROM chips when the development of the O.S. was fully finished.
ST systems had several dedicated coprocessors for interrupt handling (MFP 68901), sound (the old Yamaha YM 2149, same as General Instruments AY-3-8910 found in MSX computers, Amstrad or Oric), video (a custom chip called "Shifter"), and memory (a custom chip called "GLUE" and the MMU).