GIMP is an acronym for GNU Image Manipulation Program. Now, I now what you're thinking..."huh?". I had the same reponse, and I'll admit I had to look up the meaning of GNU (a computer operating system comprised entirely of free software). Basically, this means that GIMP, despite it's rather unfortunate name, is a super-duper, Photoshop like, photo editing software that is completely free. That's right, free!
The program does require a quick download to your computer, and it offers many of the same capapabilities of Photoshop to create dynamic, layered images, without the large price tag. Now, is it going to do everything Photoshop can do? No, but if you're interested in more advanced photo editing, this is a great free tool to start with before moving on to the big ticket programs.
Now, despite how great GIMP can be for budding visual artists, it is not the easiest program to learn. There is a very brief online manual available. I would even suggest checking out a few books about Photoshop, since several of the functions availabe in GIMP mirror those in Photoshop.
Now, go forth and photo edit!