One of the best strategy games to ever come out on PC.
In StarCraft you get to control three races: Terrans, Zerg, and Protoss. The Terrans are your future high-tech humans, who have a great deal of colonies and influence in the StarCraft Universe. They are probably the most versatile race in the game, which combines power and defence together. The Zerg are your "nasty, slimy creatures" that you know are the bad guys from the first time you see them. Although their units are weaker than those of the other races, their strength is that they can spawn in incredible numbers, and outnumber any enemy. The Protoss are your psionic-powered high-tech aliens, which would seem like the most powerful race, and in strength they are. Unfortunately, they have very expensive units compared to the other races, which makes them weak in numbers.
Overall, I think this game is one of the best to come in a long time, and it should have been impossible for you not to have at least heard of this game. After 9 years, it's still fun popular, and that's not only what makes a great game, that's what makes a classic.