Skype is a software application that allows users to make voice and video calls and 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, file transfer, and videoconferencing. Skype has 663 million registered users as of September 2011. The network is operated by Microsoft, which has its Skype division headquarters in Luxembourg. On 10 May 2011, Microsoft Corporation agreed to acquire Skype Communications, for US$8.5 billion.