If you are interested in photography you will probably already know what a RAW image file is. If not, put simply it is an unprocessed image from your camera that represents the "as shot" state.
You can of course have your camera do the processing for you and apply things such as; white balance, sharpening and exposure. If you want to have greater creative control over your images, shoot in RAW rather than JPG and process the images on your computer rather than in the camera.
There are now a plethora of different software packages available to process RAW files, each offering different features and capabilities.
UFRaw or the Unidentified Flying Raw is a free (GNU General Public License) utility that allows you to process RAW image files out of your camera. It can be used as a stand alone application or as a GIMP Plugin.