DVD is compatible with most other optical media storage (but there is a distinction between DVD and DVD-ROM below:
VCD resolution is 352x288 for PAL and 352x240 for NTSC. The way most DVD players and Video CD players deal with the difference is to chop off the extra lines or add blank lines. When playing PAL VCDs, the Panasonic and RCA NTSC players apparently cut the entire 48 lines (17bottom and none off the top. The Sony looks better. [Does anyone know if it cuts 24 lines off the top and the bottom, or if it scales the full picture to fit?]
Most DVD-ROM computers will be able to play Video CDs (with the right software), since its already possible with current-model CD-ROM computers.
Note: Many Asian VCDs achieve two soundtracks by putting one language on the left channel and another on the right. They will be mixed together into babel on a stereo system unless you adjust the balance to get only one channel.
However, Pioneer and Samsung have announced combo players that will play laserdiscs and DVDs (and also CDVs and audio CDs). Denon is rumored to have an LD/DVD player in the works also.