H.323 is a set of standards from the ITU-T, which defines a set of protocols to provide audio and visual communication over a computer network.
H.323 is a relatively old protocol and is currently being superseded by SIP (Session Initiation Protocol). One of the advantages of SIP is that its much less complex and resembles the HTTP/SMTP protocols.
Therefore most VoIP equipment available today follow the SIP standard. Older VoIP equipment though would follow H.323.