The story of Kingdom Hearts surrounds the hearts of people connecting in the Realms of Light and Dark.
In Kingdom Hearts the main areas for worlds are the Light Realm, Dark Realm and the Between Realm. There are many, many worlds in the Light Realm, and each of those worlds make up the stars in the sky.
Realm of Light
The world where people live which is overflowing with light. There was a time long ago when the world was one, but it was destroyed and separated into countless small worlds. Each world is covered in a barrier which cannot be seen by the naked eye, and without particular forms of travel it is normally not possible to travel between worlds. On top of that, traveling between worlds without concern for each individual world could possibly cause disorder and thus it is fundamentally not allowed.
Each world contains a “Heart of the World” and through that a door can be opened which will break the wall of dark power, turning pieces of the wall into shooting stars which come hurling down and can be used as material for “Gummi Ships.”
In attempts to prevent disorder in other worlds, Donald and Goofy change out of their Disney Castle royal clothing and hide their origins during their journey.
A virtual space world constructed from data. Aside from the world King Mickey reconstructs from the data from the first Jiminy journal in KH coded, the world DiZ created for Roxas’s fake summer vacation of Twilight Town in KH2 and others also exist.
The Twilight Town in which Roxas spent his time was inside a data world in the computer connected to the pod in which Sora slept.
In KH1, once Sora defeated Ansem, many of the worlds swallowed by darkness were revived, but a few of them remained dormant and sleeping. These became the basis for KH3D, the worlds trapped in slumber. Seeking out the Sleeping Keyhole and using the Keyblade on it will awaken each world and bring it back on the path to being reclaimed.
Travel between worlds
Each world in the Realm of Light floats in the place called the Sea of Stars or the Dimensional Sea. Sora and friends use the Gummi Ships to open up special corridors and travel between worlds.
A ship used to cross the sea of stars, developed at Disney Castle. It is made up of pieces of the walls of worlds (Gummi) and runs on energy from smiles.
From the outside the Gummi Ship may look like a toy, but it has the ability to cross the sea of stars and fight evenly with ships the Heartless use.
Broken pieces of worlds’ walls which become shooting stars. King Mickey in KH BbS took a star shard from the Mysterious Tower in order to travel between worlds.
Between Corridor / Corridor of Darkness
One way to travel between worlds is using the Between Corridor, which a Keyblade wielder can open and utilize using the Keyblade’s power. Not only that, but those shrouded in darkness can also use dark powers to open corridors, differentiated being known as Corridors of Darkness. Going through one of these corridors, the heart and body of a person are eaten away at by the darkness, so it is even dangerous for those shrouded in darkness to use.
Terra, Aqua and Ventus were able to open the corridor gates which Eraqus provided and travel through them on their transformed Keyblades.
The black coats of Organization XIII, DiZ’s red cape and the armor which Terra, Aqua and Ventus wore all help protect them as they travel through the corridors.
Realm of Darkness
A different realm which is on the opposite side of the Realm of Light. Generally people do not live here, but it is filled with Heartless and creatures of darkness.
The doors which connect the Realm of Light and the Realm of Darkness are the Door of Darkness and the Door of Light. Only those from the Realm of Light may pass through the Door of Light, and only those from the Realm of Darkness may pass through the Door of Darkness.
When Ansem took over the body of Riku, his heart was sent into the Realm of Darkness and even after regaining his body, he was stuck there.
The realm which exists between that of Light and Dark. There are fewer worlds than those which exist in the Realm of Light. Also, the space there is unstable and entrances to the dark corridors may suddenly open.
Where worlds are placed:
Realm of Light – Radiant Garden (Hollow Bastion), Destiny Islands, Disney Castle
Between (Close to Light) – Mysterious Tower
Between (Mid Close to Light) – Twilight Town
Between (Mid Close to Dark) – Castle Oblivion
Between (Close to Dark) – The World that Never Was
Realm of Darkness – Dark Margin