There's also a major difference in the development between the two software packages.
gimp.org creates the GIMP software. They have the ideology of free software as in freedom of speech. They feel users could and should be involved with every step of the development including developing the software itself. GIMP does not take donations and any time donations are offered they are redirected to donate to GNOME or the Free Software Foundation. This truly is the methodology of free and open source software.
gimpshop.com takes the software from gimp.org, and customizes it to have an interface more like Adobe Photoshop. This way people coming from the Photoshop world have an easier time adjusting to GIMP. It doesn't add any functionality it just makes some design decisions on the UI and maybe some plugins to include which aren't default. gimpshop.com also partners with companies and offers 3rd party software which they make money from people trying out and they also accept donations. This helps pay their server costs and whatever else they use it for. I'm not sure if they contribute development upstream to gimp.org at all (they're not required to).
One thing to remember, without the gimp.org GIMP software then gimpshop wouldn't exist. I don't quite like how gimpshop encourages users to try 3rd party software "in order to keep gimpshop free". That's a personal thing though.
In general, I find it is more useful to tell someone up front exactly what GIMP is about. It doesn't help someone to say, "It's just like Photoshop but free!" First and foremost, the user will have expectations of it being exactly like Photoshop and anywhere the software diverges from that idea would be seen as a flaw in the software compared to Photoshop. GIMP follows an entirely different set of goals for what they want to get out of an image manipulation software so they have created an identity of their own. It's best to say that anything which can be done in Photoshop can also be accomplished using GIMP but they are different software. If you want something "just like" Photoshop then get Photoshop. Otherwise GIMP provides a competitive alternative which can provide a suite of tools to produce professional quality work.
Hopefully I didn't rant too much. Don't misunderstand, I'm not saying gimpshop is a bad piece of software. I've used it before and had a good experience. They just have different goals than the GIMP project and it's good to understand that is the case.
Last edited by sagteck
on Sun Jun 17, 2012 11:47 pm, edited 1 time in total.