The so-called "content container" is purely a device for centering things on the page. It is necessary because of a bug in IE 5, which is still used by about 5% of the population as of 2006. Inside the content container, we have a navigation bar on the left (outlined in green) and a "right container" (outlined in blue).
The navigation bar is an unordered list that has been stylized so that it looks more-or-less like buttons. You can pretty much click anywhere inside the button and get the link to fire.
Site Index
The sole purpose of the right container is to keep the main content (inside the lime box) centered in the available space to the right of the navigation bar. This is another accomodation for an IE bug. Actually, it's not so much a bug as a lack of standards compliant features.
The primary content goes inside the "content" div (outlined in lime). In a small screen it will stretch to be the size of the right container. In a larger screen, it will have a fixed width and be centered in the right container.
The purpose of the horizontal rule is to make sure we've cleared the navigation bar should the main content be minimal. In a good browser, the horizontal rule won't show up. In IE it does and therefore, makes things uglier. I'll put up with some ugliness so that I don't have to do (and later undo) hacks to accommodate IEs outdatedness.