From Wikipedia

Adobe Photoshop, or simply Photoshop, is a graphics editing program developed and published by Adobe Systems. It is the current and primary market leader  for commercial bitmap and image manipulation, and is theflagship product of Adobe Systems. It has been described as “an industry standard for graphics professionals”  and was one of the early “killer applications” on the PC.

Adobe’s 2005 “Creative Suite” rebranding led to Adobe Photoshop 8′s renaming to Adobe Photoshop CS. Thus, Adobe Photoshop CS4 is the 11th major release of Adobe Photoshop. The CS rebranding also resulted in Adobe offering numerous software packages containing multiple Adobe programs for a reduced price. Adobe Photoshop is included in most of Adobe’s Creative Suite offerings.

Due to its popularity, “photoshop” has become both a verb to refer to photo editing and a genericized trademark for graphics editing software. Photoshop’s popularity, combined with its high retail price, makes Photoshop’s piracy raterelatively high.  Adobe countered by including SafeCast DRM starting with Adobe Photoshop CS.

Development

During this time, John traveled to Silicon Valley and gave a demonstration of the program to engineers at Apple Computer Inc. andRussell Brown, art director at Adobe. Both showings were successful, and Adobe decided to purchase the license to distribute in September 1988. While John worked on plug-ins in California, Thomas remained in Ann Arbor writing program code. Photoshop 1.0 was released in 1990 for Macintosh exclusively.

Features

Photoshop has strong ties with other Adobe software for media editing, animation, and authoring. Files in Photoshop’s native format, .PSD, can be exported to and from Adobe Illustrator, Adobe Premiere Pro, After Effects, and Adobe Encore to make professional standard DVDs and provide non-linear editing and special effects services, such as backgrounds, textures, and so on, for television, film, and the Web. For example, Photoshop CS broadly supports making menus and buttons forDVDs. For .PSD files exported as a menu or button, it only needs to have layers, nested in layer sets with a cuing format, and Adobe Encore DVD reads them as buttons or menus. Photoshop is a pixel-based image editor, unlike Adobe Illustrator, which is a vector-based image editor.

Photoshop can utilize the color models RGB, lab, CMYK, grayscale, binary bitmap, and duotone. Photoshop has the ability to read and write raster and vector image formats such as .EPS, .PNG, .GIF, .JPEG, and Fireworks. It also has several native file formats:

• The .PSD (Photoshop Document) format stores an image with support for most imaging options available in Photoshop. These include layers with masks, color spaces, ICC profiles, transparency, text, alpha channels and spot colors, clipping paths, and duotone settings. This is in contrast to many other file formats (e.g. .EPS or .GIF) that restrict content to provide streamlined, predictable functionality. Photoshop’s popularity means that the .PSD format is widely used, and it is supported to some extent by most competing software.
• The .PSB (Large Document Format) format is a newer version of .PSD designed for files over 2 gigabytes.
• The .PDD (PhotoDeluxe Document) format is a version of .PSD that only supports the features found in the discontinued PhotoDeluxe software.

CS3

   

Smart Layers display the filter without altering the original image (here on Mac OS X)

Photoshop CS3 is marketed with three main components of improvement over previous versions: “Work more productively, edit with unrivalled power, and composite with breakthrough tools.” New features propagating productivity include streamlined interface, improved Camera Raw, better control over print options, enhanced PDF support, and better management with Adobe Bridge. Editing tools new to CS3 are the Clone Source palette and nondestructive Smart Filters, and other features such as the Brightness/Contrast adjustment and Vanishing Point module were enhanced. The Black and White adjustment option improves users control over manual grayscale conversions with a dialog box similar to that of Channel Mixer. Compositing is assisted with Photoshop’s new Quick Selection and Refine Edge tools and improved.. image stitching technology.

CS3 Extended contains all features of CS3 plus tools for editing and importing some 3D graphics file formats, enhancing video, and comprehensive image analysis tools, utilizing MATLAB integration and DICOM file support.

CS4

Photoshop CS4 features additions such as the ability to paint directly on 3D models, wrap 2D images around 3D shapes, convert gradient maps to 3D objects, add depth to layers and text, get print-quality output with the new ray-tracing rendering engine, and enjoy exporting to supported common 3D formats; the new Adjustment and Mask Panels; Content-aware scaling (also known as seam carving); Fluid Canvas Rotation and File display options. On 30 April, Adobe released Photoshop CS4 Extended, which includes all the same features of Adobe Photoshop CS4 with the addition of capabilities for scientific imaging, 3D, and high end film and video users. The successor to Photoshop CS3, Photoshop CS4, is the first 64-bit Photoshop on consumer computers.

 
Try the Adobe Photoshop CS4 here.