The Dreaming was the creation of Neil Gaiman for his series The Sandman as the dimension in which all dreams and imagination exist, ruled over by Dream of the Endless.
The origins can be traced back to Jack Kirby however, in his initial 70's series The Sandman, a theme he had been developing before in titles such as The Strange World of Your Dreams and Spirit World. His was a more sci-fi themed dimension described as;
"Somewhere, between heaven and earth, there is a place where dreams are monitored. This is the domain of a legendary figure, eternal and immortal, who shares with man and beasts all the secrets of the ages".
In Kirby's version, The Sandman was a superhero figure protecting children from nightmares. Although his series was short lived, Neil Gaiman restructured his version around the similar concept. Writer Roy Thomas revealed that The Sandman was actually Dr. Garret Sandford and had become trapped in the Dream Dimension, eventually going insane and committing suicide. In Gaiman's Sandman series, the nightmare demons Brute and Glob were said to have created false world inside the mind of Jed Walker in Morpheus' absence and tricked humans, including Garret Sanford and Hector Hall, into thinking they were the actual Sandman.
"The Dreaming is the realm and place of power of Dream of the Endless, the place in which all dreams reside and where dreamers go when they're dreaming. "The Dreamworld is infinite, although it is bounded on every side", there are two gates in the Dreaming: the gates of Horn and the gates of Ivory, those gates were carved by Morpheus himself when the world was young to set some order. "The dreams that pass through the gates of ivory are lies, figments, and deceptions. The other admits the truth".
Morpheus has been described as an incarnation of The Dreaming and there's a direct relation between both beings, as such, it accommodates to Dream's wishes and it is what he wants it to be, it's also affected by his feelings and condition, that's why when he was imprisoned, The Dreaming was collapsing as he was. Morpheus can also create portals to The Dreaming in the waking world and portals in the Dreaming to the waking world; he can cross between both worlds; he can create and destroy beings or places. To sum up, he can do whatever he wants in The Dreaming.
In the heart of the dreaming there lies Dream's castle, the front gate of which is watched over by a Gryphon, a Wyvern and a Hippogriff. Within the castle of Morpheus there's a Gallery, a place in all of the Endless' realms in which they keep the sigils of their siblings that can be used to communicate with one another or to travel from one realm to another. There's also a Library guarded by Lucien in which are saved all of the books containing the stories that have ever been dreamed, volumes that do not exist in the waking world.
Other areas of the Dreaming include Abel's House of Secrets and Cain's House of Mystery, both located in the frontiers of nightmare, Eve's cave and Fiddler's Green before his demise at the hands of The Furies.
The Dreaming can be affected by many factors, but one of the more dangerous is a vortex. A vortex is a real person who somehow becomes the center of The Dreaming, it destroys the barriers between dreams making it gather into one, which eventually collapses upon itself and damages The Dreaming beyond repair. There's at least one vortex every era and Morpheus has to prevent any damage to The Dreaming by destroying it; it's the only time when he has the power to take a human's life. It has been said by Dream that once he failed in destroying a vortex and a whole world was lost. One of the vortexes that posed a threat to The Dreaming was Rose Walker.
The Dreaming is inhabited by dreams, nightmares, places (such as Fiddler's Green) and other entities. According to a census taken by Lucien, the Dreaming's librarian, there were about eleven thousand and sixty-two creatures living in the dreaming, however, the amount is constantly changing for some of them are destroyed and others created. Some of the more relevant are listed below:
The House of Mystery
The House of Secrets
The Fashion Thing