"GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc."
Check out the features listed on their website at: http://www.gimp.org/about/introduction.html
They also have some decent HTML tutorials on a variety of topics at: http://www.gimp.org/tutorials/