VOIP = Voice Over Internet Protocol
In simple terms it means using the internet instead of the standard phone system cables for connecting the most expensive bit of the call - normally the international part. (In fact most PSTN calls have been connected like this by the phone companies for years and we have therefore been ripped off for years as this bit should be free if you are connected to the internet)
It means a bit of fiddling about as its not perfect technology yet. But for anyone prepared to invest a little time to understand it and make it work for them, there are some definite potential savings.
VoipCheap (
www.voipcheap.com) for example gives unlimited free calls to the UK, Ireland and Netherlands and Voipbuster (
www.voipbuster.com) currently offer free calls to around 20 destinations worldwide. If you use their web application then you need to run a PC and buy a headset or USB connected phone. There are ways of avoiding this inconvenience by they all cost more money and could outweigh your savings if you usage is small.
Its not for everyone but it is getting better known and it does present some interesting options (incoming numbers in other countries for example).
There are some good starter resources here ...
http://www.voip-info.org/tiki-index.php