It's a game that simulates the evolution of a lethal virus from a commutable germ into a worldwide, lethal pandemic. You have 200 days to kill the human population before they find a cure for you. Each day, your infection spreads and the death toll begins to mount. Easy, right?
Not so much. You've got to stay one step ahead of those pesky humans-- for one thing, if you kill too many people too fast, worldwide panic will shut down international airports-- and your chance to spread to other continents. Success depends on how cleverly you're able to evolve your virus.
The game is ridiculously simple and seems tedious at first glance, but it's more challenging than it sounds, and there are an infinite number of ways that you can wipe out the world population. Pandemic is a perfect way to spend a cynical afternoon.
*(I think, technically, it's "virii," but that sounds stupid.)
(UPDATE: It's not "virii". I'm right! Thanks, Phalligator)