Post processing in Photoshop (and similar tools) are one of the benefits of digital photography. With quite simple means you can edit the files from your camera, improving the final product. For some it's almost as important as taking the actual shot.
There are many ways to learn "photoshopping" (or retouching). I cannot direct you to a particular web site but I know many books covering this subject. There are books on portrait retouching, HDR and of cause general editing in Photoshop.
Scott Kelby is the number one selling author in the field of photography. You can browse through his collection of books (and books from some of his colleges/friends) on Kelby Training Books.
Of course there are web sites too, but if you would like to learn the art of photo editing I would recommend a book.