H.323 is an umbrella recommendation from the ITU-T, that defines the protocols to provide audiovisual communication sessions on any packet networks. It is currently implemented by various Internet real time applications as NetMeeting, Voxilla (for Linux). It is a part of the H.32X series of protocols which also address communications over ISDN and PSTN. A challenger to H.323 is SIP a standard from IETF. Both protocols are used in Voice_over_IP (VoIP) (a.k.a., InternetTelephony or IPTelephony).
H.323 to describe the system, the call model and call signaling procedures.
H.225 protocol is used to describe the media (audio and video), the stream packetization, media stream synchronization and control message formats.
the H.245 describes the messages and procedures used for opening and closing logical channels for audio, video and data, capability exchange, control and indications.
