An NEF fils is a raw photo captured by a Nikon digital camera. It is saved in an uncompressed format that stores data exactly as it was captured by the CCD. NEF files also contains image metadata, which includes the camera ID, lens that was used, and other settings.
The NEF format supports 12-bit or 14-bit data, depending on the camera used to capture the image. Any adjustments to image settings such as exposure, hue, tone, sharpening, and white balance can be made with software once the image is saved to a computer. These changes to the image are stored as instruction sets separate from the raw image data in the NEF file, which means that no matter how many edits are applied to the image you can revert back to the original.