NEF is a raw file format produced by the Nikon D1 digital camera. Instead of containing a fully processed image like a TIFF or JPG file, NEF files store data directly sampled from its CCD. This format, like other raw formats, requires post-processing to be made viewable. The advantage of the NEF format is that it retains all 12 bits of brightness data from the camera CCD.
NOTE: This file format requires the DigiCam plug-in, only available in the professional edition of ThumbsPlus.