StarCraft was the first space-based RTS game Blizzard Entertainment published back in 1998.
It has a lot of influences in gameplay from WarCraft II, but has it's own gameplay, and three unique races, rather than two mirrored ones.
StarCraft revolutionised gaming in Korea, and brought significant changes to how we perceive RTS games in the West. The game is still played somewhat regularly in the West, and is still hugely popular in Korea and Asia.
You can read more about StarCraft Brood War and StarCraft II at the SCraftWiki.com, or the StarcraftWire.net site.