Isengard is a fortress to the east of Rohan, where the tower Orthanc stands. It is an important location in The Lord of the Rings, particularly during The Two Towers, when Saruman lived at Orthanc and commanded armies of Orcs.
Historically, Isengard was one of Gondor's three fortresses, with Orthanc holding one of the Palantíri. It stood beside the river Isen, which watered many forests and fields within the valley. Before the trilogy begins, Saruman came to power there, and was corrupted by Sauron through Orthanc's Palantír. Unknowing, Gandalf came to him during The Fellowship of the Ring for advice, but was imprisoned at Orthanc until his rescue by Gwaihir.
His allegiance revealed, Saruman dammed the Isen, logged most of the valley's trees, and dug deep pits into the earth. These pits were used for creating the Uruk-hai, among other preparations for war against the Rohirrim. Meriadoc Brandybuck, Peregrin Took and Treebeard came across the ravaged forests of the valley, which upset Treebeard and the other Ents enough to attack Isengard. The Ents brought down the main walls of the fortress and flooded the pits by destroying the Isen's dam.
Saruman and Gríma Wormtongue remained inside Orthanc, but were finally captured after the arrival of Gandalf and Théoden. In the Peter Jackson movies, Gríma killed Saruman at this point instead of later in the Shire. The Ents reclaimed the valley after Sauron's defeat, planting a whole new forest there.
Watch also for the nearby Fangorn Forest being transplanted into Isengard's valley, or for Isengard and/or Orthanc to be moved into Fangorn.