There are many Internet telephony applications available. Some, like CoolTalk and NetMeeting, come bundled with popular Web browsers. Others are stand-alone products. VoIP also is referred to as Internet telephony, IP telephony, or Voice over the Internet (VOI)

See "The Difference Between VoIP and PSTN Systems" in the Did You Know... section of Webopedia.
Also see the VoIP Meets WiFi article in the Did You Know? section of Webopedia.

Enterprise VoIP Planet
The IT Manager's Guide to Voice over IP

Healthy VoIP Nets - Quality of Experience
A new process, called Quality of Experience, endeavors to go beyond voice to include video and IPTV connections, plus taking into account the effect that the end-user applications may have on the end-to-end connection.

Securing VoIP Networks, Part 1
SBCs are network elements that are deployed at the border between packet-based networks to manage the signaling and media messages that support Internet multimedia services (for example, voice or video). For example, they can be placed between service providers in a peering configuration or in an access network that provides VoIP service to residential or enterprise customers.

Six Steps to Getting Your Network Ready for Voice over IP
Using low bandwidth CODECs helps reduce bandwidth requirements however due to the overhead of IP, UDP and RTP protocols, such reduction is limited.

What SMBs Need to Know About VoIP
Replacing existing internal phone systems with VoIP networks can cut costs, simplify administration and enable a variety of new applications that improve collaboration and productivity.