Skype is a software application that allows users to make voice and video calls and chats 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 was founded by Niklas Zennstrom and Janus Friis who were also the founders of the file sharing application Kazaa.

Skype has also become popular for its additional features which include instant messaging, file transfer, and video conferencing. Skype has 663 million registered users as of 2010.

Unlike other VoIP services, Skype is a peer-to-peer system rather than a client–server system, and makes use of background processing on computers running Skype software; the original name proposed – Sky peer-to-peer – reflects this.

Some network administrators have banned Skype on corporate, government, home, and education networks, citing reasons such as inappropriate usage of resources, excessive bandwidth usage, and security concerns.

On 10 May 2011, Microsoft Corporation agreed to acquire Skype Communications, US$8.5 billion. The company is to be incorporated as a division of Microsoft, and Microsoft will acquire all of the company’s technologies, including Skype, with the purchase.