DVD is high bitrate (~11mbps) MPEG2. VCD is MPEG1.
DVD resolution is 720x480. VCD is, at most, 480x480. (NTSC. PAL is x576)
DVD audio is Dolby Digital 5.1 and many other sound formats, such as DTS, SDDS, 2.1 stereo, etc. DVD can also have multilanguage audio and selectable subtitles. VCD is 2-channel MPEG layer 2.
DVD discs can hold MUCH more data than CDs. DVD can be up to 18GB, CDs are only 650-700MB.
VCDs are usually many discs in length. Rarely is a VCD movie ever on a single CD. If it is, it is probably a very short movie or very bad quality because of the amount of compression needed to fit a whole movie on a single CD. DVDs, however, are only one disc for the main feature.
The most important difference is the picture and sound quality. The difference is astounding. If you have any doubts, compare it for yourself.
Also, beware of bootlegs. There is no copy protection in the VCD format, so piracy is fairly common.