Skype is a peer to peer voice service enabling people around the world to talk together at a quality similar to that of a telephone line. Calls made from skype to skype are free, whilst the service also offers the ability to call abroad for free for a cheap price.
Fixed in the latest version are various security flaws enabling a hacker to crash the system. Other fixes include making the ring sound stop when a call is answered and allowing skype passwords to be remembered.