Skype is a software application allowing users to make voice, video calls, and to chat over the Internet. Calls to other users WITHIN the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include instant messaging, videoconferencing, and file transfer.
For more information on Skype, check out the official Skype website.