The Nikon EM was a model of Nikon 35mm camera offered for sale in the late 70's and early 80's.
It was an experiment by Nikon to produce consumer-level cameras that would offer good cost/performance, unlike their professional-level cameras, which gave good performance at huge prices.
Like most cameras of the time, the Nikon EM is a manual focus, manual winding camera. It has center-weighted metering, but offered only aperture-priority operation. This means that the user selects the lens aperature, and the camera would use the metering information to select an appropriate shutter speed. A manual override shutter speed of 1/60 sec., and B (bulb) operation was also available.
A dedicated winder, (EW, I believe) was also available. This winder, when attached to the EM, would wind the film after each shot was taken. The user would still have to rewind the film manually at the end of the roll.
Concurrent to the introduction of the Nikon EM, a series of consumer-grade lenses were offered: the Series E. These lenses had the same high-quality glass lens elements, but cut cost by replacing the sturdy metal lens tube with plastic parts that were easily assembled by machines. Although generally thought to be inferior to the Nikkor lenses, these Series E lenses had their fans due to their reduced weight. The 75-150 zoom, and the 100mm portrait lens in this series are particularly well regarded.
Still regarded as a reasonable back-up camera to a Nikon setup, because it is fairly rugged, cheap, and compatible with nearly all Nikon gear.