The GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages.
GIMP is a great open source application, but some users feel it’s not very user-friendly, specially if you used Photoshop before. That is why Scott Moschella hacked GIMP to look and feel like Photoshop.