A raw digital image is the data read directly from an imaging sensor that has not been processed by a converter. Photographers often write RAW in capital letters, as they do JPEG—although strictly speaking raw is not an acronym. RAW converters have to interpret the image data properly in order to convert them to a usable bitmap file formats such as TIFF and JPEG, for example in demosaicing the original image.
Raw is often referred to as a digital negative. A raw image has higher bit depth than JPEG (allowing more gradations to be recovered from highlights and shadows), lossless compression, and it has not had any sharpening or noise-reduction "baked in" as is the case with a camera's own JPEG output. Raw files cannot be manipulated directly in a traditional image editor, unless it has a RAW converter. For file extensions, Nikon uses .NEF, Canon .CRW, .CR2, .CR3, Pentax .PEF, Olympus .ORF and many others. In the case of some camera models, "raw" files may have been modified in-camera with noise reduction or distortion correction, in which case it becomes debatable whether the name is truly accurate.
While there is no universal, industry-standard raw format, Adobe has offered an open standard called DNG which is available for any camera manufacturer to use (and several camera-makers do offer this as an option when saving images). Different sensors may produce differing data, so cameras from the same manufacturer could also produce RAW file data that are not compatible.