Cave-citadel in western Beleriand, ruled by Finrod who was known as Felagund, the master of caves, because of it. Like Gondolin, it was established as a hidden city, and was based on dwarven architecture. It was mostly underground.
Forces from Nargothrond were heavily involved in Dagor Bragollach, with the leading warrior Gelmir captured by Morgoth. Only a small company took part in Nirnaeth Arnoediad, but, led by Gwindor, they played a central role, charging rashly into battle and precipitating disaster.
The reason there was only a small force is that Nargothrond was on bad terms with the sons of Feanor, two of whom Celegorm and Curufin had usurped rule of Nargothrond during Finrod's absence (he was imprisoned in Tol Sirion along with Beren, and who died there). The rightful ruler Orodreth was eventually restored with the help of Beren and Luthien.
Turin later travelled to Nargothrond along with Gwindor, and precipitated the downfall of the city by counselling a more aggressive military strategy (the elves had previously relied on secrecy and arrows in the dark). A bridge was built to allow assaults, but instead it was used by Glaurung and a host of orcs to attack Nargothrond and lay it to waste.
Glaurung turned Nargothrond into his lair until he was slain; after which Mim the petty dwarf took it over, until slain by Hurin.
The name means "halls/caves of the Narog", because it was located near the river Narog.