NEF is a RAW file format by Nikon. It is modeled on TIFF files used in printing/publishing. NEF supports 12/14-bit data depending on camera. Edits to NEF files are instructions separate from raw data. You can revert to original.
To open NEF files, use Nikon’s free Capture NX-D software for Windows/Mac. Other RAW editors like Photoshop/Lightroom also open NEF. NEF files contain metadata about lens, settings, etc. RAW files capture lighting, intensity and colors seen by the camera sensor.
Changes to a NEF file alter instructions, not the image. Multiple NEF edits don’t affect image quality. Nikon has details on the NEF format. Ensure files are actually NEF before opening.
NEF files are usually Nikon camera RAW images. Many editors like Photoshop/ACDSee open them. Nikon bundles ViewNX and Capture NX-D software to open NEF files. NEF files can be converted to other formats for editing and sharing.
Adobe Photoshop
Nikon Capture NX-D
GIMP