Best reference I can find is this
http://www.dvdrhelp.com/vcd
http://www.dvdrhelp.com/svcd
SVCD uses MPEG2 video instead of MPEG1 (often the difference between free capabilities and those that need the pro/plus/pluging added)
SVCD uses higher resolution.
SVCD can use a higher bitrate (tradeoff against capacity)
SVCD can do higher quality, can trade quality for time.
The downsides? - slower encoding, and not as compatible - some DVD's will not play SVCD (but might be persuaded to using the HEADER TRICK)
