One of the names the Otherworld goes by is Elfhame, as it is home to the Fae. Elfhame is right next door to our world, which some of the more ancient Fae sometimes call Manheim?. Places in Elfhame often correspond directly to places in Manheim, and some of the more powerful Fae can cross directly between them. However, the majority of the Faeries require a point of access. These can be direct, as in a faerie ring, or indirect, as a passage through the Between, marked with the sign of Thereamid.
The original document is available at http://www.thereamid.com/tiki/Otherworld