Format NEF is graphical format developed by Nikon. Its extension NEF means Nikon Electronic Format. In fact it is RAW file, but Nikon has edited it for their own purpose. Its main advantage is high quality, which goes hand in hand with huge size of files. Best known sign of these files is that setting of camera is saved separately. In practice it means that the picture alone is saved without modifications, but in the same file are separately saved settings of this camera. That allows to simply edit photos, which can be achieved in few easy steps.
Company Nikon used format .nrw, but today is mostly used newer format known as .nef. Difference between them isn´t big, just there is the advantage of using by application Nero, which is burning application.
There is severity of programs and applications that can open these files. Nikon on its own developed program Nikon ViewNX, then we can mention Adobe Photoshop by Adobe Systems. Company Microsoft modified their Picture Viewer, so even he can open these files. Google has also its own application, its Picasa that can open files saved in this file format. The best choice how to open these files is program Able Rawer.