Adobe Photoshop is a graphics editing application popular for it's extensive amount of features. Photoshop is also, currently, the leading graphics editing application. 
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Photo shop is also an image creation software as well as an editor. Photo shop can create any effect or style needed in a drawing or painting or layout. There are graphic software that can do specialized work faster and more efficient than Photo shop (such as painter for realistic paint effects), but Photo shop can do it all in one program.
The basics of Photo shop are easy to learn, even the CS versions. They are very intuitive, and there are several ways to do almost everything to work with an individual's style of drawing and skill level yet you can spend years learning all the pro level features.
Photo shop works by altering individual pixels in an image as opposed to a vector drawing program that draws with points, lines and objects mathematically. Photo shop is best with images that have complex textures, blends and photo realism, but Photo shop is also very good at vector drawing as long as the image doesn't need to be scaled and you don't need specialized CAD drawing tools.