Both are image manipulation software widely used all across the globe both by amateurs and professionals alike. Gimp and Photoshop are both the type of softwaresa that used in the process of enhancement or editing of images whether it involves changing the image or distort it, adding elements to it and even save the image in other available formats. There is no definite difference between GIMP and Photoshop since both are more or less the same in most ways. One cannot say as to which one is more user friendly because both seem to have the same features except for the fact that Photoshop is the more widely used software and thus is more well known. Plus, Photoshop CS4 [current available version] comes with basic and extended version where basic version have all previous CS3 features while extended version have some advanced features like 3d rendering which is perfect for those who uses AutoCAD, Maya or Rhino software before. Simply put, Adobe Photoshop CS4 extended version is like a software suite ranging from image manipulation software, animation software [Flash] and 3d rendering software [CAD]. Of course, extended version will cost more to purchase it but those who bought the basic version can upgrade their software to extended version for a small fee. Currently, Adobe Photoshop CS3 is about $599 each while CS4 is costs at $699 and its extended version is sold at $999 each.
GIMP on the other hand is a newly introduced. Well, not really new since it first makes a debut on 1996 but for most people, GIMP is almost unheard of. What good of GIMP when Photoshop already has it all? Like Linux, GIMP is free and still improving day by day. Yes, GIMP is free open source software and can be downloaded for free at its website. Though initially GIMP is built for Linux operating system but now it has version or patch which makes it usable for Windows and Macintosh [Apple] operating systems. Still, since Photoshop is widely used and have created its own formats like .psd, .pdf and so on; most organizations rely heavily on these formats and want to save their projects in these formats hence Photoshop is the choice to be. GIMP can't save projects on these formats because they are already patented by Adobe and cannot be used by other software. Worry not, though GIMP can't save in those formats, GIMP is able to open or read those formats but have to save it in other formats.
Photoshop has years and years of history behinds it thus adding to its integrity and credibility. Just look at its versions released previously from version 1 to CS4 while GMP is till new where currently, its stable version is 2.6.8. If a choice is to be make, most will choose Photoshop but please do give GIMP a try before buying a costly one. GIMP is free and it won't hurt a dime to try using it, and experiment it. As stated earlier, though Photoshop can save into psd and pdf formats while GIMP can't; GIMP can still save the projects into other widely use formats like gif, jpg or jpeg or even png. In terms of its appearance, GIMP and Photoshop looks almost the same. The features which Photoshop basic versions have are also available in GIMP such as path, layers, brush, image distortions, brightness etc.
Those who hunt for quality should target the Photoshop software as GIMP has a bit-depth limitation at 8-bit making it basically an unlikely choice for professional use.Photoshop is the first to have 64bit version while other are still in 32bit version making it not only first choice for professional use,but also the fact that it is compatible with newer generation of computers. Having said that it is important to note that if adjustments are made in layers , the limitation of being 8-bit seems less obvious as it results in the stacking of corrections that are almost as good as Photoshop.
Gimp also seems to have a disadvtange when compared to photoshop with trivila issues like its menu layout to more serious problems like the fact it lacks the color management feature and does not offer adjustment layers.If you would like to enhance GIMP , the availabilities of plug-ins like extensions, add-ons as well as snap-ins allows one to add on features that are vital for their specific needs.This makes it an advantage for the GIMP user as they are able to use its modifiable or fluid nature to adapt to ones preferences making it simple . GIMP seems to be rising as a favorite for avid but amateur photographers that want to use the software to enhance their work while keeping it simple. The GIMP manual allows the user to obtain the help they need easily.
What it comes down to is simple as eventually personal preference,capabilities and budget is the main issue when coming to choose between the softwares.