The Twilight Realm is inhabited by the mysterious Twili. It is notable that normal enemies in the Twilight Realm appear in shades of black and in some cases (notably the Kargarok) are drastically altered in appearance.
The government of the twilight realm is a monarchy. The latest known Queen was Midna.
How to enter
Although the method is not clear, it was very clear that with Zant's help, Ganondorf could travel between realms without either of these devices.