A movement in British rock. To get the full picture you have to understand that the 1976/77 punk explosion drove a lot of mainstream rock bands underground, many of them didn't survive. However the fanbase remained and in pubs and clubs in working class areas such as East London, Newcastle on Tyne and the West Midlands new bands started to form taking on the musician ship of the older 'dinosaur' rock outfits, but using the new punk method of doing things; brash attitudes, agressive sounds, self management / control etc.
The punk movement in the UK started to run out of steam as a mainstream musical form in the summer of 1979 and the new metal bands started to emerge from the pubs and clubs. Early examples were Saxon, Magnum and Raven. They were joined in 1980 by acts that were to become household names - Iron Maiden, Def Leppard etc.
NWOBHM was a very broad church that embrassed art rock (Pallas / Haze), glam (Wrathchild / Tygertailz), Early death and proto-thrash (Venom / Warfare) etc, as well as countless denim and leather bands. (Angelwitch / Demon)
The excepted period for NWOBHM is summer 1979 to autumn 1985. After this point NWOBHM became a tag to refer to bands in the Iron Maiden / Judas Preist vien (Marshall Law / Strangeways / Idol Rich)
The style has since influenced most modern forms of rock and metal.