Doriath was the greatest Sindarin kingdom in Beleriand during the First Age. Founded during the latter part of the Years of the Trees, it was at first called Eglador ('Land of the Forsaken') for those Teleri who were followers of Elwë, later known as Elu Thingol, that stayed behind in Middle-earth looking for him.
At the center this kingdom was Menegroth, the great underground abode of Thingol and Melian. It was carved by the Dwarves of Belegost, with whom Thingol had good realtions with.
After the First War of Beleriand, Melian wove a protection spell around the kingdom. The spell, called the Girdle of Melian, encircled the forests of Neldoreth to the north, Region to the south, and the woods of Nivrim to the west. The Girdle led to the kingdom being renamed Doriath ('Fenced Land') and prevented entry to those who did not have Thingol's permission to do so. All Sindar were permitted to enter the lands of Doriath, as were dwarves, and those Noldor who belonged to the house of Finarfin. But those Noldor of Fëanor's house were forbidden to enter the hidden realm, as a result of their part in the kinslaying along the shores of Alqualondë. Thingol also forbade the use of Quenya within the lands of Doriath.
The Girdle of Melian made Doriath a safe haven for the Sindar throughout all the wars that took place in Beleriand, until Thingol was slain by the Dwarves of Narog for a Silmaril in FA 502. After Thingol's death, Melian departed forever from Middle-earth, breaking the protective spell she had woven around the kingdom thus allowing it to be invaded. The kingdom of Doriath was eventually destroyed by Fëanor's sons in FA 509.
(Map this page excerpted from "The Atlas of Middle-earth" by Karen Wynn Fonstad)