The Dreaming is one of the many realms in the DC/Vertigo universe, though primarily affiliated with Vertigo's characters. The Dreaming is effectively where all dreams come from and take place. Though usually humans are unaware that they have actually traveled to The Dreaming, some unique individuals do become aware and, as such, are able to interact with the realm's inhabitants, including Cain and Abel, the biblical brothers. It is also connected to House of Mystery and the Dreamtime where the minor god Rainbow Serpent resides.
The Dreaming is a vast realm, ruled over by Dream of the Endless. In The Dreaming, Dream has virtually infinite power and control. Though he rarely chooses to venture out and deal with the many souls traveling back and forth from The Dreaming, he does occasionally interact with some individuals.
Along with Gemworld, Faerie, Hell, and Heaven, among others, The Dreaming is one of the largest and most influential of the realms. In the Vertigo miniseries "The Books of Magic", Doctor Occult takes young Timothy Hunter to visit the many different magical realms, including The Dreaming. There, the young boy encounters Dream of the Endless and shares a few words with him.