It can be used as a simple paint program, an expert quality photo retouching program, an on-line batch processing system, a mass production image renderer, an image format converter, etc.
GIMP is extremely expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
Not only is the program itself freely available, but so is the source code. This means that anybody can contribute features and bug fixes to the program (and many people have).
The home page for GIMP is here. Note that I have no affiliation with the GIMP project, please direct any questions to the GIMP team.
I've put some GIMP examples on my web-site here: standard GIMP logos and animations.