Glossary:
VoIP
VoIP is usually called Voice over IP (where IP means Internet Protocol).
VoIP is the collection of technologies that allow telephone calls to be made via the Internet rather than using the traditional PSTN telecoms network.
The challenges of delivering Voice over IP are due to the nature of TCP/IP packet based networks. IP networks do not guarantee that data will arrive in the same order they were sent. This isn't a problem for general file transfer but for voice data it is crucial. The problems that face VoIP are latency and jitter and affect the quality of the call.