Beleriand was an area of Middle-earth, existing from the time of the Years of the Trees through the First Age, that lay west of the Ered Luin. The land was divided into East and West sections by the mighty river Sirion that ran from Eithel Sirion in the north to the Bay of Balar in the south.
Beleriand was originally home to the Sindar who dwelled mainly in the lands of Doriath and the Falas along the west coast. Later the Laiquendi would pass over the Ered Luin and settle themselves just west of the mountains in a land called Ossiriand. The exiled Noldor dwelled mostly to the north in areas above the Ered Wethrin and Ered Gorgoroth. Those who followed Finrod Felgund, however, settled to the south in West Beleriand. Eventually, the race of Men known as the Edain would call Beleriand home as well, some befriending and living with the Noldor.
The land would not see much peace during the First Age as Morgoth and his forces were ever in battle against the elves, mostly the Noldor. Eventually Beleriand was destroyed at the end of the First Age after the Great Battle, which was also known as the War of Wrath. The only remaining lands that exisited from the Second Age on were a remnant of those that belonged to the area of Ossiriand and which were called Lindon.
Map of Beleriand, from "The Atlas of Middle-earth" by Karen Wynn Fonstad