Well photoshop can pretty much do everything
I use photoshop to design my websites (the graphics) i use it to touch up photos (cropping, color etc..)
I use it for everything haha
You can download it off of Adobe.com (makers of it) or get it shipped to your house (if you have slow internet do that)
It has all the things you can think of so i cant really tell you everything about the program because theres to much to say! you may want to check out
Adobe for more info
And.. If you get it and want to learn how to do stuff you can go to pixel2life.com or goodtutorials.com for tutorials on how to do stuff
(i personally dont like tutorials! but they are nice to learn where things are and what they do)