Adobe Photoshop is primarily a raster, or bitmap, graphics and image editing program with limited vector capability. It is the de facto standard for still image processing and manipulation across industries as diverse as advertising, geology, medicine and astronomy.
Adobe Photoshop is used by professionals and amateurs alike for its powerful editing capabilities. It is now released in two professional editions: Photoshop Standard, which contains the latest iterations of its traditional still image editing tools, and Photoshop Extended, which adds video, 3D and scientific/medical imaging capabilities. A consumer version (Photoshop Elements) is not built by the same engineering team that works on Photoshop, but there is a certain amount of cross-pollination between them.
Photoshop 7 was the last individually release-numbered version. Photoshop 8 was released as "Photoshop CS," recognizing its role as part of the Adobe Creative Suite (CS). Current versions of Adobe's Photoshop are named with their CS version, so that Adobe Photoshop CS5 is version 12.
Raster images are composed of a rectangular grid of points ("pixels" -- a contraction of "picture elements") where each point is assigned a color and brightness value. Raster graphics are resolution-dependent. They cannot scale up to an arbitrary resolution without loss of apparent quality.