How Composite Video Works

So I was doing some browsing around on Youtube the other day and I starter to wonder, how does composite video work? I read a handful of Webpages, pretty much all of which said the exact same thing more or less. So i was wondering if anyone knew how composite video works to generate an image with color and everything.

Please use simple words so that it is easy to grasp (or if you DO use complex terms like "harmonics" please describe what the term means).

I know that the signal level determines the color but how do you do things like pixel selection which is necessary to generate images?

Did you try Google or better Wikipedia directly?
Analog video is really quite well documented on the web but to really understand it you need to have some background knowledge - or read even more to complete the info.