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. A few are also available in-game and others through events. Click here for a list of all available Pokémon
One of the new features in the game is the introduction of a new slot of abilities that almost each and every Pokémon have. Many of these are given over the Dream World on the Global Link, but there are a couple of Pokémon within the games who will automatically have the abilities.
Normally, you would be unable to get these abilities through usual breeding. However, if the female Pokémon has the Dream World ability, there is a high chance that the ability of the offspring will be the ability it has listed for Dream World abilities. However, if the male Pokémon has the Dream World ability and the female doesn't, and if you breed the female with a Ditto, the Pokémon will definitely not get the Dream World ability.
This will allow you to continue to breed the Pokémon to get the perfect nature and Egg Moves that you require for your team.