Adobe Photoshop is an image editor. Any digital image, be it a photo you shot or an image you pulled from online, can be edited, tweaked, played with, etc. in this program. There is an enormous amount of features and things you can do with Photoshop, much more than could be included in one research guide, so further references and lists of books on Photoshop have been added for you to explore.
The best way to learn this powerful editor is to work with it. Get in and play around with an image you want to edit. Following the tutorials and actually editing an image will really help all these techniques stick and make basic processes second nature.