Each land is a Swamp in addition to its other land types.
4/1/2008 If Urborg, Tomb of Yawgmoth and either Blood Moon or Magus of the Moon are on the battlefield, all nonbasic lands will be Mountains, not Swamps. Urborg, Tomb of Yawgmoth will be a Mountain as well. It will have “: Add ” and no other abilities.
7/18/2014 Urborg, Tomb of Yawgmoth isn’t a Swamp while it’s not on the battlefield.
7/18/2014 Urborg’s ability causes each land on the battlefield to have the land type Swamp. Each land thus has the ability “: Add .” Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they’re legendary or basic.
7/18/2014 If Urborg loses its abilities (for example, if it becomes a creature and then Turn to Frog targets it), all lands on the battlefield, including Urborg, will still be Swamps, but Urborg won’t have the ability “Each land is a Swamp in addition to its other land types.” Urborg also won’t be able to tap to produce , but other lands (including those that enter the battlefield later in the turn) will. The way continuous effects work, Urborg’s type-changing ability is applied before the effect that removes both the type-changing ability and its own mana ability.