Although the two cassettes look virtually the same, that's about the only things they have in common (they both use half-inch wide video tape in the same-sized cassette shells). S-VHS uses different carrier frequencies to record the luminance portion of the video signal (where all the picture detail is). This higher carrier frequency means that almost double the picture detail than VHS is recorded, and with less noise and picture distortions. Because of the technical differences, only specal VCRs can play S-VHS video recordings. Also, S-VHS video tape has a different chemical formulation than VHS.
In general, S-VHS can reproduce an image with a higher quality than the average television set is capable of showing. Compare that to VHS which always produces a picture quality worse than the worst of mediocre television sets.
We use S-VHS tapes and equipment for mastering most of our video production work, because it's more than good enough for most jobs which end up on VHS or DVD for playback. Using greater, and more expensive, formats would be a waste of money (yours and mine). And many of recording formats don't have the continuous recording length that we need for some jobs. DVD is unsuitable as a mastering format, it uses heavy compression, needs re-encoding (and will usually be recompressed) if you edit it, has a delayed start-up time, isn't very robust, and has many compatibility problems. The format used for production and mastering doesn't have to match the format used for end playback, but should be better than it.
VHS resolution is about 230 horizontal lines.
S-VHS resolution is around 400 horizontal lines, or greater.
Most S-VHS VCRs can play both types of tapes, but only a few VHS VCRs can play S-VHS tapes.
S-VHS tape can record up to three hours, continously,
on a standard cassette.
Four-hour S-VHS tapes are available, but the quality of such tapes aren't as good.
MiniDV and Digital 8 only record 60 minutes in standard play mode, or 90 minutes in a long play mode (which tends to bring about a lot of compatibility problems).
BetaCAM camcorder tapes are even shorter, and is a completely different system than the domestic Beta VCRs.