Voice over Internet Protocol (VoIP) allows the use of the internet as the transmission medium for telephone calls by sending voice data in packets using IP rather than by traditional circuit transmissions of the PSTN.
The biggest advantage of VoIP is that telephone calls over the internet do not incur any charge beyond what is paid for internet access, much in the same way as not having to pay for individual emails sent over the Internet. This is true of all calls, whether they be to a next door neighbor or somebody on the other side of the planet.