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 runs on all major operating systems.

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.

Ada Ivanoff from SitePoint has put together a wonderful blog post which should help you with GIMP no matter your skill level.

Here is the Index :

1. 101 Tutorial for Beginners: Learning the Basics
2. GIMP for Beginners – Lesson 1 – Getting around GIMP
3. The GIMP Toolbox(PDF)
4. Mixing Colors in GIMP
5. Cutting Out An Image Using GIMP
6. Using the QuickMask In GIMP
7. Understanding Layer Masks in GIMP
8. Use GIMP Layers to Create Amazing Photos(PDF)
9. Work With Paths in GIMP
10. Mastering GIMP Filters
11. Controlling Lighting & Shade in GIMP
12. How to Give Your Photos that ‘WOW’ Factor(PDF)
13. Colorize a Black and White Photo with GIMP
14. Photo Retouching with GIMP
15. Creating a Custom Brushes in GIMP
16. Introduction to GIMP Animation
17. Using the Cage Transform Tool in GIMP
18. Introduction to GIMP Scripting
19. A Collection of GIMP Text Effects Tutorials