The Atari ST is a home/personal computer that was released by the Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals.
Part of the next generation of home computers, the ST battled against the Amiga for supremacy only for both to lose out to the PC. Never a direct rival with the CPC, the ST was seen as more of the next logical step to take in your home computing habit and no doubt many former CPC users made the jump to Atari's machine.
For its time, it was an interesting 68000-based computer. It was cheaper than the Amiga and there were no color Macintoshes. 2.1 million units sold (USA only?) according to wikipedia, other sources claim up to 6 million units sold worldwide.
For more details on the Atari ST, head over to the official Wikipaedia page where you will find all the contact details and links you need to get you started in your investigations!