Some people love Skype; others are less enthusiastic.
Basically, if you're connected to the internet, you're constantly sending and receiving data to and from it. If your friend is also connected to the internet, so is she. Neither of you are paying anything (other than your normal ISP subscription fees) for doing so. All that Skype does is to let you send data (i.e. your voice) to the internet, so that it can be forwarded to your friend (and, of course, vice versa).
Click on 'See how easy it is to make free calls with Skype', here, for some pretty pictures to explain it further:
http://www.skype.com/...skype-to-skype-calls/
Chris