It is a graphic design program that allow you to "manipulate" image and such.
Better explanation is already post at the first reply.
Here is the link for official sites and MSRP price (some seller is cheaper) :
http://www.adobe.com/products/photoshop/
But if you are totally new and with limited budget, and want to create fun stuff without professional output, here is the option:
http://www.adobe.com/products/photoshopelmac/index.html
I use both and sometimes the features on element is easier and you can get some result within two steps in elements while you have to do at least 4 steps to get similar result on photoshop.
Don't get me wrong, the element also have some profesional output, and powerful enough, and your imagination is the limit for both of them, but for starter and budget-wise I prefer you try element first, and you can step-up if you are more familiar, have the budget and ready to make money with photoshop.
Good Luck