Skype is a software application

Application software

Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...

 that allows users to make voice and video calls and chat over the Internet

Voice over IP

Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...

. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phone

Mobile phone

A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s can be made for a fee using a debit

Debit

Debit and credit are the two aspects of every financial transaction. Their use and implication is the fundamental concept in the double-entry bookkeeping system, in which every debit transaction must have a corresponding credit transaction and vice versa.Debits and credits are a system of notation...

-based user account system. Skype has also become popular for its additional features which include instant messaging

Instant messaging

Instant Messaging is a form of real-time direct text-based chatting communication in push mode between two or more people using personal computers or other devices, along with shared clients. The user's text is conveyed over a network, such as the Internet...

, file transfer

File transfer

File transfer is a generic term for the act of transmitting files over a computer network or the Internet. There are numerous ways and protocols to transfer files over a network. Computers which provide a file transfer service are often called file servers. Depending on the client's perspective the...

, and videoconferencing

Videoconferencing

Videoconferencing is the conduct of a videoconference by a set of telecommunication technologies which allow two or more locations to interact via two-way video and audio transmissions simultaneously...

. Competitors include SIP

SIP

-Business and economics:* Sales Incentive Plan and Share Incentive Plan, employee motivation schemes* Systematic Investment Plan, investment strategy...

 H.323

H.323

H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

 based free software Empathy

Empathy

Empathy is the capacity to recognize and, to some extent, share feelings that are being experienced by another sapient or semi-sapient being. Someone may need to have a certain amount of empathy before they are able to feel compassion. The English word was coined in 1909 by E.B...

, Linphone

Linphone

Linphone is a VoIP application available on PCs running Linux or Windows, Apple computers running Mac OS X, and Android and iPhone mobile phones. It uses the Session Initiation Protocol for communication and is licensed under the GNU General Public License. Linphone uses GTK+ for GUI and on Linux...

, Ekiga

Ekiga

Ekiga /i k ai g a/ is a VoIP and video conferencing application for GNOME and Windows. It is distributed as free software under the terms of the GNU General Public License. It was the default VoIP client in Ubuntu until October 2009, when it was replaced by Empathy...

 http://ostatic.com/blog/bye-bye-skype-top-3-free-replacements but primarily the Google Talk

Google Talk

Google Talk is a freeware voice over Internet protocol client application offered by Google Inc. The first beta version of the program was released on August 24, 2005...

 interface supported by Google Chrome

Google Chrome

Google Chrome is a web browser developed by Google that uses the WebKit layout engine. It was first released as a beta version for Microsoft Windows on September 2, 2008, and the public stable release was on December 11, 2008. The name is derived from the graphical user interface frame, or...

 and released as the WebRTC

WebRTC

WebRTC is a framework that was open sourced on June 1, 2011 that allows web browsers to conduct real-time video chat.Its inclusion in the World Wide Web Consortium standards is supported by Google, Mozilla and Opera....

 standard proposal is expected to provide Skype's long term competition, as it is based on the same codec technology (developed by Global IP solutions) as Skype itself.

Skype has 663 million registered users as of September 2011.