StarCraft is, without a doubt, one of the best computer games ever, at least as far as I'm concerned. Not just among real-time strategy games or science fiction games.
Speaking of that, Starcraft came in at #7 on the IGN.com Top 100 Games list. Considering that this list includes video games as well as PC games, #7 is quite an accomplishment! (BTW, the #1 game overall is Super Mario Bros. and the #1 PC game is Civilization II.) There is no shame at all in coming second to Civ II; that game is another one of my favorites. :D
Now what is Starcraft, exatcly? First off, StarCraft is a real-time strategy game, which means that all players are performing game actions simultaneously. [This is opposed to turn-based games, in which one player performs all his game actions, ends his turn, and then the next player performs his game actions, etc. etc.]
StarCraft is set far in the future, in the 2400's. The game takes place in a sector on the edge of the galaxy. The game features three races: Terran, Protoss and Zerg. Terrans are human colonists, and the Terran army features the kind of units you'd expect to find in a human army: tanks, guys carrying rifles, one-man fighters, covert operatives, medics, etcetera. The Protoss are an extremely advanced alien race, and their units are expensive but powerful. The Zerg race consists of many horrific monsters. They use no technology, but their powerful warrior breeds can easily match advanced Terran and Protoss technology.
A more detailed description of each race (coming soon, at least in theory):
Terran
Protoss
Zerg
The race feature is one of the great things about StarCraft- all three of the races are totally different- yet Blizzard made them completely balanced.
Another one of StarCraft's great features is the AMAZING storyline-most of which is set up in the singleplayer campaigns. StarCraft comes with 3 campaigns, one per race. Throughout each campaign, you command units of that race, against various enemies. The missions in each campaign get harder as you go along. [An additional three campaigns come with BroodWar.]
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Note: I got this Battlecruiser pic from the StarCraft Fan Art section.