SVCD stands for Super Video CD (called also SuperVCD or Chaoji VCD). It is a new CD standard (actually it is currently undergoing official standardization process) developed in 1998 by Chinese consumer electronics manufacturers, Chinese government and VCD consortium (Sony, Philips, Matsushita and JVC).
SVCD is a successor for extremely popular video format called VideoCD which was based on MPEG-1 video encoding. SVCD itself contains MPEG-2 video stream and MPEG-1 or MPEG-2 audio stream (MPEG-1 stereo audio layer II, MPEG-2 stereo audio layer II or MPEG-2 Multi-Channel 5.1 surround audio). It's video bitrate is normally higher than VideoCD's -- clear difference to VideoCD is the fact that SVCD doesn't specify a certain bitrate for video. Unspecified video bitrate also causes a situation where one SVCD disc can contain various amount of video -- normally one SVCD disc contains 35-40 minutes of video, but by reducing the bitrate, one CD can hold up to 74 minutes of video (which is the same amount what VCD disc contains).
As an addition, SVCD can also contain multiple audio streams (just like a DVD-Video can), subtitles, still images, multi-level hierarchical menus, chapters (for indexing), hyperlinks and playlists.
Just like VCDs (and audio CDs), SVCDs require a specific way how they are burned on the CD -- just sticking all the required files into CD structure doesn't make disc a SVCD compatible. Most of the new CD burning applications support SVCD already, so authoring your own SVCDs should be relatively easy.
SVCD's resolution is 2x higher than VCD's, in PAL the resolution is 480x576 and in NTSC it is 480x480. Framerates are 25fps and 29.97fps, just like in any other video format. SVCD's quality is somewhere between VideoCD and DVD-Video. Most of the DVD players can play SVCD discs which makes is perfect format for backing up your DVD movies and a very good alternative for DivX ;-) format.
SVCD has also gained popularity among movie studios -- in Far East distribution, of course -- and most of the studios already release their movies in SVCD format in China, Hong Kong, Taiwan, etc...