Voice over IP is exactly what its name suggests. It is voice communication that is transmitted over an Internet Protocol (IP) service.
VoIP is seen as a disruptive technology. For a century, people have been happy to use the plain old telephone system (POTS) to make calls. When POTS calls are made, a circuit opens between the two callers. No matter how long they speak, or whether there are quiet periods in the conversation, that circuit remains open between the caller and receiver. Importantly, the further a caller is from a receiver, the dearer the call cost because telecommunication companies must lease a longer line for that call to take place.
VoIP changes this model. Now a call, just like an audio file or image file, can be broken up into packets of data and sent over an IP network. The catch with this is packets might not always travel the same path to get to their intended destination. So the continual challenge for a VoIP service provider is that these packets arrive quickly and as one voice stream when they get to the other end. Any hiccups along this path and your conversation literally makes no sense.
To get VoIP in your home or office all you need is a Broadband connection to the Internet, a subscription to a VoIP service provider and a client -- an analogue telephone adapter (ATA), VoIP Phone or "soft phone".