You can also have a look at the illustrated features overview.
This is only a very quickly thrown together list of GIMP features. But basically the same code also runs on Windows and macOS. GIMP is written and developed under X11 on UNIX platforms. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted. It is designed to be augmented with plug-ins and extensions to do just about anything. 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. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. GIMP is an acronym for GNU Image Manipulation Program.
