VoIP Standards to know
The common standards are Session Initiation Protocol or SIP, H.324, H.323 and proprietary solutions - the most know being Skype.
H.323 and H.324 are multimedia standards designed to provide converged services - video telephony services, voice and data.
SIP is the standard for basic voice over IP calls - Vonage desktop uses SIP. There are several standardization groups working on bringing these standards together.
Often for a home user, proprietary solutions like Skype are the only solution. Small and large business will go with SIP, H323 or H324 depending on their needs (Voice, Video and Data).