Skype is one way into Voice Over IP - that is telephone calls over the internet. But the downside of Skype is that they use proprietary protocols so it's great to call another Skype user or to use their "Skype Out" facility to call from your PC to a land line anywhere in the world. But you can't easily connect to users on other VOIP networks and you get no choice but to pay Skype's own "Skype out" rates. If, on the oter hand, you use the same sort of hardware but the software you use is SIP/RTP compatible, these are open internet protocols for VOIP and any suppliers using them should be compatible with your hardware and software. So you get to choose from lots of different SIP providers and if you want to call an Aunt in Australia or a brother in Hong Kong you can use one SIP supplier for one call and another for the other call and just cherry pick the best rates available.
So, while "Skype" may be "king of the hill" today (not least because Ebay bought them!) they are going to have to do something pretty soon about inter-operability and SIP/RTP conformance or they risk being left behind now that all the big telcos like BT, Telefonica, France Telecom, Belgacom, et al. are starting to roll out their big SIP/RTP networks and (quite importantly) offer guaranteed end-t-end QOS (quality of service - to guarantee crackle free calling!)
Cliff
(who kind of does this stuff for a living)