The Umbra is the term for spirit realms in the classic World of Darkness. Use this tag for questions about the Umbra, the Penumbra (Earth's Spirit Realm), and the Gauntlet separating the Earth from the Penumbra.
In the world design of the World of Darkness, the real world (tellurian) floats in the Umbra, surrounded by its own Penumbra.
The Penumbra is the part of the Umbra that is a reflection of Earth, and which is separated from it through the Gauntlet. It pretty much acts as a bridge between the deeper Umbra and the real world.
Were-beings and Mages are the most common ones to access the Umbra to call upon spirits and travel through it. Wraiths and Mummies know it as the Underworld.