Is an image file that contains minimally processed data from image sensor (scanner, camera, video). They are not ready to be printed with a bitmap editor but preserves most of the information of the captured image.
Raw formats are often reffered to as the "digital negative" as it fundamentally works in the same way as negatives in film photography. The RAW file is not directly usable, but needs to convert the information to create an image. The RAW format contains a wider dynamic range of colour. The purpose is to save, with minimum loss of information, data obtained from the sensor, and the conditions surrounding the capturing of the image (the metadata).