Quoting Wikipedia's short, but accurate description of GIMP:

"GIMP (short for the GNU Image Manipulation Program) is a free software raster graphics editor. It is primarily employed as an image retouching and editing tool and is freely available in versions tailored for most popular operating systems, including but not limited to Microsoft's Windows, Apple's Mac OS X, and GNU/Linux. In addition to detailed image retouching and free-form drawing, GIMP can accomplish essential image editing tasks such as resizing, editing, and cropping photos, photo montages combining multiple images, and converting between different image formats. GIMP can also be used to create basic animated images in the GIF format. GIMP's product vision is that GIMP is or will become a free software high-end graphics application for the editing and creation of original images, icons, graphical elements of web pages and art for user interface elements."