I can't believe there's disagreement between the people actually answering but...
A hard court is medium everything.
Clay courts are bit 'stickier' on the ball. It makes the ball bounce higher, and it slows the ball down. Spins are carried a lot more. A good baseline game tends to be more effective because when trying to put the ball away from the net, it's easier for your opponent to chase the ball down and pass you.
Grass courts are a lot slicker, the ball bounces lower and faster. It's good to have a big serve and a good net game on grass.
Carpets are inconsistent and depend on what kind of surface they're covering, they usually come in between hard and grass.
I couldn't tell that much difference between hard, carpet, and grass in virtua tennis, but on clay the ball moves noticeably slower. But for all of them the winning strategy is basically the same, I think it's more important to play to your character's strengths than to the court's strength.