Within the Dream World, you'll occassionally encounter a variety of Pokémon. These Pokémon are not typically available within the Unova region and thus are mostly Pokémon that were previously native to Kanto, Johto, Hoenn & Sinnoh. These Pokémon, once you have befriended them, will then be able to be encountered within the Entralink Forest, accessed through the C Gear. In this area, you will be able to capture them with great ease being able to select the ball to capture them. When you arrive, a Dream Ball will appear in your pack. This will guarantee capture, but you can attempt capture with different balls.
Many of the Pokémon within the Dream World will have abilities that the Pokémon you transfer over from Diamond, Pearl, Platinum, HeartGold & SoulSilver don't normally have, such as Vaporeon with Hydration and Glaceon with Ice Body. Click here for a list of all Dream World abilities