Voice over internet protocol (VOIP) technology is the use of internet protocol (IP) networks (data networks of the type developed for the internet) to carry voice phone calls.
The most obvious use of VOIP for consumers is to use internet connections to make voice phone calls through a computer. This is usually cheaper, but less reliable, than the use of a standard phone line.
More important, although much less visible, is the move by telecoms companies from networks that handled voice and data separately (demanding two separate networks) to unified IP networks that handle both. These networks, unlike consumer VOIP technology, can guarantee the quality of voice calls. They reduce costs by allowing telecoms companies to move to a single network, rather than maintaining separate voice and data networks.
VOIP is also being built into telephones and many corporate internal networks are already using VOIP, for much the same reasons as the telecoms companies. Ultimately all voice calls are expected to be carried over IP networks end-to-end.