Can't open RAW pictures on my computer?

So I use a Nikon D...okay, screw it, I don't remember the name. But when I take pictures and select the formatting, if I choose RAW, it won't open on PhotoShop. I read that RAW is the best format for editing pictures, but then why can't I open it on PhotoShop? (I have CS2, and I don't have the money for CS4.) Thank you!