Adobe Photoshop has been the industry-standard image editing software for two decades now, and is used by professional photographers, artists and designers all over the world. Despite the best efforts of competitors such as Corel Paint Shop Pro, Apple's iPhoto, and the open source GIMP, there really isn't anything that comes close to Adobe's flagship product in terms of features, performance or quality.
There isn't much that comes close to the price either; the full version of Photoshop CS5 for Windows PCs costs over £600, while Photoshop CS5 Extended (which adds video and 3D handling functions) costs close to £900, and the whole Creative Suite 5 package, which includes Photoshop, Illustrator, InDesign, Dreamweaver, Acrobat, Fireworks and more, can cost over £1,600 depending on which version you choose. If you already have Photoshop CS2, CS3 or CS4 you can get the upgrade package for around £185, which is still expensive but slightly more affordable. Since it is always an issue that gets mentioned in the comments section, I'll also register my protest at the usual Dollars-for-Pounds price equivalence. In the USA the PC version of Photoshop CS5 costs around $600, which is roughly £400 in real money. I've read Adobe's official explanation for the massive price differential, but it doesn't take any of the sting out of being charged 50 percent more for the same product.
CS5 is the twelfth version of Photoshop. After 20 years of development and this many versions, most programmers would have run out of ideas, and updates would just be minor patches. Not so for Adobe though; this latest version of Photoshop contains many new features and major improvements, certainly enough to make users of CS4 seriously consider an upgrade. The new feature everyone is talking about is of course the amazing Content-Aware Fill, but other equally significant improvements include sophisticated new selection tools, improved HDR image creation, new painting tools, automatic lens correction, the novel Puppet Warp feature, and perhaps most importantly for photographers, vastly improved Camera Raw processing, including new noise reduction algorithms.