Graphical User Interface editor for bitmap images from Adobe Systems Inc. The de facto standard for graphics professionals from roughly the mid-'nineties to the present (2008).
Notable features of Photoshop include:
It's mid-priced, by the standards of professional applications. Some early competitors to Photoshop priced themselves out of mass adoption by even the pros. On the other hand, it's too expensive for casual users and impoverished students. Partly for this reason, there's both a strong market for lite image editors (some from Adobe) and widespread piracy of the Real Thing.
Version 3 of Photoshop in 1994 introduced layers, and was the first program to make them popular. Ever since then layers have been regarded as central to Photoshop and essential for serious image work. They're often used in conjuction with paths, introduced in Photoshop 2 from 1991. (At least one competitor with layers (HSE Live Picture) came out at around the same time as Photoshop 3 but priced itself out of contention.)
Photoshop has supported third-party plugins since at least 1991.
It has the sort of facilities, like CMYK support, that are essential for people who have to worry about colour fidelity, or read and write arcane image formats, or deal with professional printers and imagers. (And those certainly aren't orthogonal problems.) As of 2008, this still sets it apart from both the lite proprietary packages (like Adobe's own Photoshop Elements) and Open Source tools like The Gimp.
Photoshop began life as an Apple Macintosh application, and it remained pre-eminently a Mac program for years after the first Windows version came out in 1992. By now (2008) that's no longer the case.
History of Photoshop
W/O photoshop, and its ilk, we would not have latter-day masterpieces like this:
boingboing.net
Alternatives
Paint Shop Pro is a less expensive, albeit lighter-weight, alternative.
Corel Painter: Not as cheap, but with an unmatched feature set. Differs from Photoshop in that while Photoshop was designed to manipulate existing images, Painter was designed to create images from scratch using a wide variety of realistic (and not-so-realistic) brushes.
The Gimp: Originally for Linux Os, The Gimp is now available on Windows and Unices (including Mac Osx). It has most of Photoshop's functionality, but is still under development. But it's free, which makes it worth it. The Gimp is limited to 8bit indexed color, greyscale, or 8bit RGB. Specifically, The Gimp doesn't do CMYK. This is the major limitation of The Gimp versus Photo Shop for print work. Other people complain about the User Interface of The Gimp.
Cine Paint: Fork off an old version of The Gimp that goes in a different direction. Aimed at film work, but with users choosing it because it is also good for video, also good for very high resolution and/or bit depth scientific imaging, or because it is faster and easier on older hardware than current versions of The Gimp are.
Gimp is good but it's not Photoshop. Photoshop still has a more complete set of tools and when the going gets tough, i.e. big files, Gimp gets left in Photoshop's dust. This is surprising; considering each programs origins, you'd think Gimp would have better memory management.
One thing in Gimp's favour though is that while it is improving, Photoshop is getting worse. Photoshop has steadily deteriorated since v5 (about the time it became a Windows app). But what can Adobe do? When you've got a perfect app you can only make it worse. In the feature frenzy they are sacrificing the best workflow in the business and I'm questioning whether the annual upgrade is worth it. - hjm
Considering Gimp's origins, it actually makes sense. It was originally written by a couple of undergrads who didn't really know what they were doing. There were some pretty glaring design errors with respect to manipulating large files; they had never really thought of that. There was a rewrite to address problems caused by this lack of foresight, but there are still 'blind spots' in the design. This makes it difficult for them to do the right thing at times, such as with smaller quantizations and colour.
The above tools all manipulate bitmapped images, such as photographs.
Is there another Wiki Page somewhere to discuss vector graphics tools, such as
Sodi Podi (SVG editor)
Ink Scape (SVG editor) (Has a wiki: wiki.inkscape.org 
)
-- David Cary
See original on c2.com