From Bulbapedia, the community-driven Pokémon encyclopedia.
Dream Clouds (Japanese ゆめぐも Dream Cloud) are a phenomenon introduced in Pokémon Dream Radar. They appear in the Interdream Zone that exists in the space between dreams and reality. Normally, they and are invisible to the naked eye and cannot be seen without the use of the Visoscope.
In the games
Dream Clouds materialize in the Interdream Zone where they produce energy. There are two variations of the Dream Cloud with different colors, one pink and one gray.
The pink clouds, when shot, will disperse and produce Dream Orbs (Japanese ゆめだま Dream Orb), a form of energy that can be used to upgrade the machine to make it easier to capture Pokémon.
The gray clouds, however, will reveal a sphere of light when shot. The sphere will begin moving around rapidly, forcing the player to shoot it with the Beam while under a time limit in order to capture it. Once captured, it will be revealed to be either a Pokémon or an item dropped by a Pokémon that can be sent into the Black 2 and White 2 games.
After certain conditions have been met, shooting a gray cloud will reveal either a Tornadus, Thundurus, or Landorus in their Therian Formes. Once captured, the Pokémon can be sent into the Black 2 and White 2 games where they can be changed into those formes with the Reveal Glass.