Technically, S-video and S-VHS are not the same thing. S-VHS (also known as Super-VHS) is a video tape recording format and S-Video refers to a method of video signal transfer that keeps the color and B/W portions of the video signal separated until it reaches the television for display or another component, such as an S-VHS VCR, DVD Recorder, or DVR for recording.
S-VHS is an "expansion" of VHS in which more picture detail (resolution) is recorded by increasing the bandwidth used for recording the video signal. As a result, S-VHS can achieve 400 lines of resolution, whereas standard VHS yields 240-250 lines of resolution.
S-VHS recordings cannot be played on a standard VHS VCR, unless the standard VHS VCR has a feature known as "Quasi-S-VHS Playback". This feature, however, only allows playback of S-VHS recordings at the standard 240-250 lines of resolution that a standard VHS VCR can reproduce. In other to get the full playback resolution of S-VHS recordings, they must be played on an S-VHS VCR.
S-VHS VCRs have both standard and S-Video connections. Although S-VHS information can be passed through standard video connections, S-Video connections can take advantage of the increased image quality of S-VHS.
In S-Video, the B/W and Color parts of the video signal are transferred through separate pins within a single cable connector. This provides better color consistency and edge quality when the image is displayed on a television or recorded on a DVD recorder or DVR with S-Video inputs, or an S-VHS VCR, which always has S-Video inputs.
Standard RCA-type Composite video connections combine the color and B/W parts of the signal, so there is more color bleeding and less contrast range than with S-Video.
The reason that S-VHS and S-video are associated with each other is that the first appearance of S-video connections was on S-VHS VCRs.
DVD players, Hi8, Digital8, and miniDV camcorders typically have S-video connections, as well as some digital cable boxes and satellite boxes,and most newer televisions, but none of these record S-VHS tapes. However, you will not find S-video connections on standard VCRs - but you will always find them on S-VHS VCRs.
Playing Standard VHS Tapes on an S-VHS VCR
That being said, although standard VHS recordings are not as high a resolution as S-VHS recordings, playing standard VHS tapes on an S-VHS VCR with S-video connections may give you a slightly better result in terms of color consistency and edge sharpness, but not in resolution. This may be visible on SP recordings, but since the quality is so poor on SLP/EP recordings to begin with, S-Video connections may not make any visible improvement on the playback of those recordings.
Why Standard VHS VCRs Don't Have S-Video Connections
The reason that standard VHS VCRs don't have S-Video connections, is that it is felt by manufacturers that the extra cost really doesn't yield enough benefit to standard VHS playback or recording to make it worth it for the consumer, especially since basic S-VHS VCRs aren't that much more expensive nowadays.