GIMP is best described as a freeware Photoshop.
There are more options that you can shake a stick at, just playing around with the filters will keep you occupied for a while. The learning curve is incredibly steep though, you will need some tutorials to figure out how to use what properly and when.
The interface is simple enough with drop down menus and some of those pesdo-windows. Click on what you want to do, and a window pops up that asks for the specifics, usually with a slide bar with a number on the side if you want to be more specific.
The startup time for the program takes a while, or it does for my computer, which is rather bad so I can't speak for everyone.
But like everything else, if you aren't the artsy type, there is no real reason to get this unless you just want to play around with all of the different options.
Review by: Shadowslayer