The dream world is a feature that will be in pokemon black and white. In it, you are able to get rare pokemon and items you couldn't normally get. Most of the pokemon have an alternate ability that is only available in the dream world. This is one of the upsides to getting it. For instance, Typhlosion's special ability is blaze, which increases his fire power when his health falls. But the dream world gives me the chance to capture one with the ability flash fire, which powers him up when he gets hit with a fire type move, as well as giving him an immunity.This opens up new opportunities and strategies for me to use with him, and this applies to any of the pokemon. You catch the pokemon in the dream world, and you can use it in the "real" world and battle with the pokemon.