H.263 is a video coding standard developed by ITU-T to specifically address video coding for low bit-rate communication. It was first designed to be utilized in H.324 based systems (PSTN and other circuit-switched network videoconferencing and videotelephony) but has since found use in H.323 (RTP/IP-based videoconferencing), H.320 (ISDN-based videoconferencing), RTSP (streaming media), and SIP (Internet conferencing) solutions as well.
H.263 was developed as an evolutionary improvement based on experience from H.261, the previous ITU-T standard for video compression, and the MPEG-1 and MPEG-2 standards. Its first version was completed in 1995 and provided a suitable replacement for H.261 at all bitrates. It was further enhanced in projects known as H.263v2 (a.k.a. H.263+ or H.263 1998) and H.263v3 (a.k.a. H.263++ or H.263 2000).
The H.263 standard is now considered primarily a legacy design (although this is a recent development). Most new video conferencing products now include H.264 as well as H.263 and H.261 capabilities.