Isengard is technically a piece of the kingdom of Gondor that Saruman the White, a wizard, is tasked with overseeing. As Gondor is without a king, and Saruman is corrupted, the wizard takes over the portion of land from which he begins his own rule in league of Sauron. Isengard transforms from a once beautiful, pristine land to one of industry and darkness.
Isengard's tower eventually goes from Saruman's place of power to his own prison after he is defeated and trapped inside. In the films Saruman is killed by his assistant Grima Wormtongue there, however in the books he takes over the Shire. Eventually Saruman is still defeated, and still betrayed by Grima.
Isengard is known for it's black tower, named Orthanc. After Saruman's defeat, Isengard was returned to Gondor where later kings would find some of Gondor's relics.