VoIP calls can be made on the Internet using a VoIP service provider and standard computer audio systems. Alternatively, some service providers support VoIP through ordinary telephones that use special adapters to connect to a home computer network. Many VoIP implementations are based on the H.323 technology standard.
VoIP offers a substantial cost savings over traditional long distance telephone calls. The main disadvantage of VoIP is a greater potential for dropped calls and degraded voice quality when the underlying network links are under heavy load.→ More - Voice over IP at About.com