Osmos is simple. You're a floaty-glowy bubble, squirting around in space. You can absorb smaller bubbles, adding their mass to yours in the process, while bigger bubbles can absorb you to death. In a representative round, the goal is to "become the biggest." Off you go, absorbing bubbles to get bigger, which allows you to absorb larger bubbles, and so on ... and then you look up and an hour has passed.
The physics are great, and the gameplay is super smooth. But the other winner is the soundtrack -- fricken excellent on headphones -- transporting you into a world of groovy bubbling grooviness. The replay value is excellent .. so if you don't have a chance to listen to a level's music the first time through, you'll have plenty of other chances when you come back for more.