ok ive used photoshop and gimpshop both before (the photoshop one was a trial >.< ) and i havent noticed anything that is different feel free to flame me if u wish but i just dont see whats so bad about gimpshop
gimp is free and yes its similar to photoshop, photoshop has more tools then gimp does and more things to do. and photoshop has brushes
gimp has brushes too i get mine from deviantart.com that site is very helpful because its the only place i could find the grungebrushes at but ya gimpshop is basically the same as photoshop (to me at least)
GIMP - GNU Image Manipulation Program (i think) its not as advanced as photoshop, and you cannot make as good stuff. its just like a more basic version of photoshop.
GIMP is the free version for people who wish to be completely legal and don't have cash to blow. I make my own brushes, so brushes aren't a problem to me. If I ever needed to, I would consider using it, but PScs works fine for me.
PS is better, but GIMP is a good practice if you can't afford PS. I used it at the beginning. ^_^ It's got alot of the same features, but also missing alot like text glow and everything ^_^