Local calls and skype to skype calls are free. There are further details on minute rates for other calls on their website. You are also able to sign up for any USA location, even if you aren't in that location and calls there would be free. I had a skype in college that was local for where my parents were and it allowed me to call them for free even though I was in another state.
I agree with itsall, you can probably get the details on their website. And I know, as the first person to answer said, skype to skype are free. Pretty much like a cell phone plan where mobile to mobile calls don't cost you any paid minutes