What is The GIMP?
The GNU Image Manipulation Program or just GIMP is a free software bitmap image editor. It also has some support for vector graphics. The project was started in 1995 by Spencer Kimball and Peter Mattis and is now maintained by a group of volunteers; it is licensed under the GNU General Public License.
The GIMP was intended as a free software replacement for Adobe Photoshop, which is the dominant bitmap editor in the printing and graphics industries.
You can download it for free from the GIMP site.
Official GIMP Site