Port and city in Northern Harad, built by evil Numenoreans in the second age and later a base for the Corsairs of Umbar (although some renditions place the Corsair base near to, rather than in, Umbar).
The city is marked on Middle Earth maps drawn by Tolkien, in contrast to most of Harad. The city name is Elvish, as befits its origin among Dunedain. (Haradrim name-forms are not given in Tolkien... I wonder why...). It also occurs in most of the Tolkien strategy games, though way outside where any action is likely to be.
Presumably fortified. Some fans have drawn detailed maps which can be found by doing a Google search - usually showing a fortified city with a number of inner levels, similar to other Numenorean cities such as Minas Tirith.
Umbar stands on the borderline between Harad and Gondor, controlling Haradrim access to the North.
The Corsairs were led into battle by Lord Nimhir of Umbar.