The best RTS ever made.
The campaign does a good job for what it is; a basic introduction to the world of StarCraft and prepares you for the fights you will be in online. You start off with the Protoss campaign, having successfully defeated the Overmind at the end of StarCraft. Tassadar fell in that mighty battle, and has become a hero of the Protoss. You lead Zeratul, a dark templar, for 8 missions, before going to the Terran campaign. Here, you follow Admiral Gerard on his quest to capture the overmind for the UED, and crush Mengsk and Raynor along the path. After effectively capturing the overmind, you play the Zerg campaign, following Kerrigan.
Once you beat or get bored of campaign, the world of Battle.Net opens to you. This is where the hundred's of hours of playtime kick in. Even once you get tired of regular matches, there are still thousands of Use Map Settings games available, each interesting, each very fun. The campaign isn't very hard or very long, but if you enjoy the game enough and play on Battle.Net, this game has taken thousands of hours of my life, easily.