Lothlórien is the Sindarin name for the Golden Wood of Middle-earth. The name translates to "Dream-flower." During the War of the Ring, it is one of the last dwelling-places of the Elves, ruled by the Lord Celeborn and Lady Galadriel. It is located east of the Misty Mountains, west of the Great River, Anduin, and on either side of the river Celebrant (or Silverlode). The Silvan Elves who live there call their land Lórien ("Dream"), and the High Elves used the name Laurenlindórenan, "Land of the Valley of Singing Gold."
Lothlórien is particularly special because of its trees, the mallorns (mellyrn in Sindarin). These trees grow to astonishing heights and are very beautiful, with silver-grey bark and golden leaves at maturity, which do not fall until spring when new leaves have grown. Furthermore, the Elves of the Golden Wood live in the mallorn trees, dwelling in high platforms known as talans (telain in Sindarin) or flets. Because of this, they are known as the Galadhrim, "Tree-people". Please note that (despite The Movies' take on the matter), the correct pronounciation is "Galathrim", not "Galadrim". Same way as it's "Kell-e-born" rather than "Sell-a-born".
The land of Lórien is an important stop for the Fellowship of the Ring, and therefore it is a big target for badfic.