For learning about video signals, Wikipedia is a useful reference, but here's a very quick summary:
Component video comprises three connections: a luma (brightness) signal "Y", and two chroma (colour) components "Pb" and "Pr.
In S-Video, there are just two signals, luma (Y) and chroma (C).
In composite video there is just one signal which is luma and chroma combined.
So you know that laserdisc stores video as a composite signal? Well, in order to display or capture this signal it needs to be decoding into its constituent parts. The first stage in this process is to separate the luma and chroma signals. This is where the comb filter, or Y/C separator, comes in. However, perfect separation is difficult. A cheap and simple comb filter may produce imperfections such as dot crawl on the boundaries between different colours, and rainbows (aka cross-colour) on black and white edge transitions.
A good 3D comb filter, such as the one in your AG1980, should reduce these effects.
To further complicate matters, some (most?) LD players do not output a pure composite signal on the yellow RCA. What they do is run the signal through the internal comb filter, do a bunch of processing (for things like DNR, RGB frame store, etc.) before sending a Y/C signal to the s-video socket. The composite video from the yellow RCA is just the same Y and C signals recombined. This means that the signal has always been put through the comb filter in the LD player (which is probably not as good as the one in your deck) and comb filter artefacts may be introduced into the video at this stage.
This is why a close A-B comparison of test patterns (or scenes with strong colours or black/white edges), is essential IMO.