Photoshop have many uses: to color correct your digital images, prepare them for printing or web, design web pages, slice pages (make hyperlinks), you can also create gif animations, work with video, 3D content (Extended version), you can also make brand new images painting or drawing them with drawing tools, also in Photoshop you can make real and unreal compositions using masking tools and many, many other things.
The Rules are:-
No additions or deletions to the subject matter of the original image. (thus changing the original content and journalistic integrity of an image)
No excessive lightening, darkening or blurring of the image. (thus misleading the viewer by disguising certain elements of an image)
No excessive colour manipulation. (thus dramatically changing the original lighting conditions of an image)