At its core, VOIP is a MEANS for transmitting voice communications (e.g., a telephone call). Virtually all telephone communications today use VOIP somewhere in the transmission path between the parties on the call. In some cases, the path will be 100% VOIP. Most of us don’t care what means are used to get our call from one place to another, we simply want the call to be connected instantaneously with high quality transmission.
In current vernacular, VOIP has come to mean the TYPE of telephone system one uses. That is, when I place or receive a call, am I using an instrument connected to a VOIP path into or from the “telephone system” or not? For example, a personal computer connected to the Internet using a service such as Skype is a VOIP connection. But, the connection on the other end may be to a “regular old” telephone connected to a landline. There are a variety of VOIP telephone systems available from using the PC and its microphone and speaker to traditional looking telephone sets connected to a “business” system.
If I’m not using a VOIP system, should I be? The answer to that question is likely tied to how you respond to the following questions:

More about VOIP from Wikipedia