Longboard wheels are typically bigger and softer than ordinary skateboard wheels. The bigger size gives more speed and the softer urethane provides better grip - both things which are good for the cruising style of longboards.
For cruising around in the streets, most skaters choose the softer wheels even though their speed and accelleration lowers slightly because of it. This is because softer wheels has better shock-absorption and therefore are more comfortable to cruise with when skating on bumpy roads.
Longboard wheels for sliding
For sliding, hard wheels are usually much better. First of all because they have less grip, meaning they slide more easily. Secondly because harder wheels are more durable, which is good for sliding where the wheels gets worn down more heavily.
Downhill longboard wheels
For downhill, it would make sense that the better choice would be hard wheels, since you can reach a higher speed with them, but most downhill skaters choose the opposite. The reason why is that when skating downhill you can reach a very high speed, making it difficult to turn without sliding. The extra grip from soft wheels helps making downhill turns easier, thus keeping you on the road when it is most dificult to stay on it.