VoIP - What is VoIP ?
VoIP, or Voice Over IP (pronounced 'voyp'), is a new technology that lets you make phone calls over the Internet.
VoIP means "Voice over IP" and it is quite simply a way of utilizing the Internet (or even in some cases intranets) for telephone conversations.
In VoIP, The primary motivations for doing so are cost and convenience as VoIP is significantly less expensive than typical telephone long distance packages, plus one high speed Internet connection in VoIP can serve for multiple phone lines.

VoIP – how does VoIP work ?
When you make a VoIP phone call, you use a normal touch-tone telephone and dial the normal phone number just as you would any other time. The people on the other end can't tell that whether you are calling from a traditional telephone or a VoIP phone.
In VoIP, The main difference is that the phone call travels over the Internet rather than through the local phone company's wires.
All you need is a Broadband Internet connection, either DSL or cable, a phone adapter, and a ordinary touch-tone telephone. You do not even need a computer.
Important device in VoIP is a Codec.
A Codec is a device or program capable of performing encoding and decoding on a digital data stream or signal
VoIP services convert your voice into a digital signal that travels over the Internet. If you are calling a regular phone number, the signal is converted to a regular telephone signal before it reaches the destination.
VoIP can allow you to make a call directly from a computer, a special VoIP phone, or a traditional phone connected to a special adapter. In addition, wireless "hot spots" in locations such as airports, parks, and cafes allow you to connect to the Internet and may enable you to use VoIP service wirelessly.