Short answers, pulled from the story.
The Atari ST was a line of personal computers from Atari Corporation and the successor to the company's 8-bit computers. The initial model, the 520ST, had a limited release in April-June 1985 and was widely available in July 1985. It was the first personal computer to ship with a bitmapped color graphical user interface using a version of Digital Research's GEM environment.
The Atari ST was designed by a small team led by Shiraz Shivji, a former Commodore engineer who had previously worked on the Commodore 64. The design was completed in five months in 1984 and was shown publicly at the January 1985 Consumer Electronics Show in Las Vegas.
The Atari ST included built-in MIDI ports as standard hardware, combined with fast and low-latency performance, making it ideal for music sequencing. Notable users included Tangerine Dream, Depeche Mode, the Pet Shop Boys, and Darude, who created his 2000 hit "Sandstorm" using Cubase on a 1040ST. The precursors to Cubase and Logic Pro both originated on the Atari ST platform.
The Atari ST ran TOS, which stood for "The Operating System" and was informally called the "Tramiel Operating System." TOS combined Digital Research's GEM graphical interface with GEMDOS, a disk operating system that provided a hierarchical file system and function calls similar to MS-DOS. Leonard Tramiel oversaw the project, known internally as "Project Jason" after developer Jason Loveman.
The Atari 1040ST, released in 1986 with 1 MB of RAM, was the first home computer to achieve a cost per kilobyte of RAM under one US dollar. BYTE magazine recognized it as the first computer to break the $1,000 per megabyte price barrier.
Dungeon Master is considered the best-selling software ever produced for the Atari ST and was developed and released on the ST first. MIDI Maze, from 1987, used the ST's MIDI ports to support networked deathmatch play for up to 16 machines. It also became standard practice in the industry to develop games on the ST first and then port them to other platforms.