VOIP is short for Voice Over IP. This refers to standard telephony or voice over computer networks.
Voice
It would only say that the voice on a normal analog telephone line is transmitted digitally (a coded sequence of 0 and 1 allows information to be transferred) is passed.
Over
Over tells you that the voice digitally pass over the Internet.
Internet
The Internet is the medium that is used to connect your connection. Of interest is that there is a continuous connection and the digital packets arrive. Otherwise, a silence on the line is the result.
Protocol
By a certain encryption / protocol through your ADSL connection data (eg e-mail, images, music files)converted into packets which will be correctly transmitted. Through this Internet Protocol is not only possible to transfer data, but also handles voice.