NEF is an abbreviation of Nikon Electronic Format. If you haven’t guessed already, it is a proprietary file format developed by the widely popular Nikon Camera. It is called a RAW file type because it retains everything captured by the Nikon camera just like any other unprocessed image file.
RAW file types are popularly known as Digital Negatives as they contain everything captured by your camera including image metadata, camera model, sensor, lens, camera settings, and other information.