VOIP stands for Voice over Internet Protocol. the VoIP also called 'IP telephony' is a set of facilities used to manage the delivery of voice information over the Internet.
It has become popular this last years mainly because of it's very low cost compared to traditional telepone operators.
If you have an Internet connection you can setup a VoIP phone service, receive and make calls through your Internet connection instead of from your usual phone operator.
VoIP allows more advanced unified communications applications than standart phone protocols, including Web and video conferencing, call waiting, forwarding, voicemail, caller ID, three-way calling, free PC-to-PC phone calls, sending data ...