As stated on
Voice over IP (also called VoIP, IP Telephony, and Internet telephony) is technology enabling routing of voice conversations over the Internet or any other IP network. The voice data flows over a general-purpose packet-switched network, instead of the traditional dedicated, circuit-switched voice transmission lines.

In this tutorial Iíll show how to use Cain to do a simple capture of half of a VoIP session. You can download Cain from: Cainís VoIP sniffing function supports the following codecs: G711 uLaw, G771 aLaw, ADPCM, DVI4, LPC, GSM610, Microsoft GSM, L16, G729, Speex, iLB and my understanding is that it is under active development so expect more to come. For more information on VoIP support in Cain see:

The Flash animation tutorial is at: