From the Developers:
"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.
"GIMP is 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.
"GIMP is written and developed under X11 on UNIX platforms. But basically the same code also runs on MS Windows and Mac OS X."
- GIMP is a bitmap/pixel based image manipulation program for photo editing and retouching, and creating images and animations.
- Offers basic and advanced image editing and retouching tools - painting, drawing and selection tools, layers and channel support, selection masks, color adjustments, paths, etc.
- Features can be extended through the use of plug-ins and scripts. Hundreds of plug-ins are available in the GIMP Plugin Registry.
- Supports a huge number of file formats for display and export.
- Supports pressure- and tilt-sensitive graphics tablets and many other hardware input devices.
- Frequently updated with new features and fixes.
- It's community developed, so users can have direct input on new features.
- Customizable and extensible through the use of plug-ins, scripts, etc.
- Runs on many platforms including GNU/Linux (i386, PPC), Microsoft Windows (XP, Vista), Mac OS X, Sun OpenSolaris, FreeBSD.
- Lacks adjustment layers, macros (actions), clipping layers, layer groups, CMYK, HDR and 16-bit per channel color support, custom shapes.
- Feature development can be slow. Some highly requested features have been "in the works" for years.
- Can be buggy. There is no quality assurance team and it's up to users to report bugs.
- Steep Learning curve; documentation is often missing or outdated.
- The cluttered interface and multiple floating windows can be off-putting for many users.
- Lacks a quality of elegance in the workflow.
- The unfortunate name makes it hard to take the program seriously.
Because the GIMP is volunteer-developed software, stability and frequency of updates could be an issue; however, the GIMP is quite mature now and generally runs without significant problems. Though powerful, the GIMP has plenty of quirks, and it's not going to be right for everyone. Windows users in particular seem to find the multiple floating windows problematic.
Since it's free and available for any platform, there is little reason not to take it for a spin. If you're willing to invest some time learning it, it can be a very good graphics tool.