VoIP stands for Voice over Internet Protocol. Using VoIP, voice information is converted into digital packets and sent over the Internet, and then converted back into analog signals before reaching the phone receiver at the other end.
With hardware based VoIP phone services, your phone has been fitted with an adapter that connects to your high-speed Internet connection. When you make a call, it goes through your local telephone company to a VoIP provider, and then over the Internet to the called party's local telephone company.
There are also software based VoIP phone services, where you use a microphone headset plugged into your computer and make calls, which are routed through your cable modem, by using the keyboard. VoIP can also be run over a private data network.
The big advantage of VoIP is that voice information sent over the Internet avoids using the fixed circuitry of traditional telephony networks avoiding the tolls charged by traditional telephone service. This is why VoIP service providers can offer features such as free long-distance calls.
The big disadvantage of VoIP is quality of service. While in theory because packets are sent over the best route at the time rather than through fixed routes, VoIP services would be reliable and consistent, in reality problems such as packet loss, bandwidth and plain old Internet outages make VoIP quality and reliability inconsistent.
VoIP is, however, the telephony of the future and the quality and reliability will improve.