A video signal is made up of five parts:
Red color information
Green color information
Blue color information
Horizontal frequency information and
Vertical frequency information
This information is combined together onto cables and in the case of a composite (yellow) connection on the back of a device all 5 aspects are combined onto a single cable. Taking the information apart will introduce some quality degradation and there is no way to provide as much of a high quality signal on a single cable. It does utilize 1 75-ohm cable.
S-Video uses 2 75-ohm cables. More cables = less signal degradation. But, S-video typically uses a mini-din connection on the end that makes people think there is something magic about the cable... Nope - it is actually TWO 75-ohm cables inside of one housing for S-video.
Component uses 3 75-ohm cables and has enough room on the cable to allow HDTV to pass over it and look phenomenal. The lower quality video will still look great though. It does not have the same issues with all the signal information being jammed onto one or two cables, but offers a lot more room for that data - so more data can be provided, or if there is not a lot of data, in can arrive more cleanly at the destination.
The red & white audio connections on the back of your VCR/DVD player etc., are not related to video. They are standard line level audio connections, and have been very standardized. Much like there is also digital audio via Toslink or S/PDIF, analog audio is a separate thing from video.
Yes, you see a lot of 3-wire RCA cables with one yellow-ended cable, one red, and one white. If it is a high quality group of 3 cables, then each one will be rated at 75-ohms and you could use that cable for component, or use two of the cables and an adapter for S-video... Or, as is often the case, you just use the yellow cable for video, then use a digital connection for your DVD player. They are grouped together for your convenience, nothing more.
More high-end A/V cable companies sell their audio cables completely separately from their video cables so you never get the yellow/red/white cable, you get one yellow cable, then a pair of red/white cables. Exactly as you need them.
75 ohm is the key - and the RCA ends are just one in a multitude of ends you can put onto a cable. BNC connections are very popular in the professional market instead of RCA connections. The cable is still a 75-ohm cable though.
Coaxial cable that comes into your home - the stuff that 'cable tv' comes in on... That is typically RG-6 cable or RG-59 cable. Well, it is also a 75-ohm cable. You can put RCA connections on the end and use it wherever you need a video cable. Works great for a very inexpensive price.
Lots of options - have fun.
