Voice over IP (VoIP, or voice over Internet Protocol) commonly refers to the communication protocols, technologies, methodologies, and transmission techniques involved in the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. Other terms commonly associated with VoIP are IP telephony, Internet telephony, voice over broadband (VoBB), broadband telephony, and broadband phone.

Internet telephony refers to communications services —voice, fax, SMS, and/or voice-messaging applications— that are transported via the Internet, rather than the public switched telephone network (PSTN). The steps involved in originating a VoIP telephone call are signaling and media channel setup, digitization of the analog voice signal, encoding, packetization, and transmission as Internet Protocol (IP) packets over a packet-switched network. On the receiving side, similar steps (usually in the reverse order) such as reception of the IP packets, decoding of the packets and digital-to-analog conversion reproduce the original voice stream. Even though IP Telephony and VoIP are terms that are used interchangeably, they are actually different; IP telephony has to do with digital telephony systems that use IP protocols for voice communication, while VoIP is actually a subset of IP Telephony. VoIP is a technology used by IP telephony as a means of transporting phone calls.

The Internet Protocol (IP) is the principal communications protocol used for relaying datagrams (also known as network packets) across an internetwork using the Internet Protocol Suite. Responsible for routing packets across network boundaries, it is the primary protocol that establishes the Internet.

IP is the primary protocol in the Internet Layer of the Internet Protocol Suite and has the task of delivering datagrams from the source host to the destination host solely based on the addresses. For this purpose, IP defines datagram structures that encapsulate the data to be delivered. It also defines addressing methods that are used to label the datagram source and destination.

Historically, IP was the connectionless datagram service in the original Transmission Control Program introduced by Vint Cerf and Bob Kahn in 1974, the other being the connection-oriented Transmission Control Protocol (TCP). The Internet Protocol Suite is therefore often referred to as TCP/IP.

The first major version of IP, Internet Protocol Version 4 (IPv4), is the dominant protocol of the internet. Its successor is Internet Protocol Version 6 (IPv6), which is increasing in use.

Voice-over (also known as off-camera or off-stage commentary) is a production technique where a voice that is not part of the narrative (non-diegetic) is used in a radio, television production, filmmaking, theatre, or other presentations. The voice-over may be spoken by someone who appears elsewhere in the production or by a specialist voice actor.

In Herman Melville's Moby-Dick (1956), Ishmael (Richard Basehart) narrates the story and sometimes comments on the action in voice-over, as does Joe Gillis (William Holden) in Sunset Boulevard (1950) and Eric Erickson (William Holden) in The Counterfeit Traitor (1962); adult Pip (John Mills) in Great Expectations (1946) and Michael York in a television remake (1974).

Voice-over technique is likewise used to give voices and personalities to animated characters. Among the most noteworthy and versatile voice actors include Mel Blanc, Daws Butler, Don Messick, Paul Frees, and June Foray.

In film, the film-maker places the sound of a human voice (or voices) over images shown on the screen that may or may not be related to the images being shown. Consequently, voice-overs are sometimes used to create ironic counterpoint. Also, sometimes they can be random voices not directly connected to the people seen on the screen. In works of fiction, the voice-over is often by a character reflecting back on his or her past, or by a person external to the story who usually has a more complete knowledge of the events in the film than the other characters.