It may help to think of a container as a skin that wraps modules. Each module on a page has to have a container and you can change the module's container in the module settings. Most containers will have the module title displayed with site's H1 style of font with a color scheme that matches the overall skin/theme for the site.  Though you can ultimately style them however you want. You can also tell a container to not display.

Containers are also .ascx files that have .css files that hold the styles for them. You can locate the container files at Portals > _default > Containers. 

Hope this helps!

Chris, I use the skin's .css file to control the width of the module(s) that are place in different panes in the skin. Though I suppose you may be able to use the container's files to control the width... I've never done it that way (or tried to).

Don't think of it as a module taking up 1/3 of the "content pane". Think of it as having 3 panes... a left-pane, center-pane, and a right-pane that exist in your skin and you can span them across the page. You can do this by floating the panes to the left and giving them whatever width you want in your skin.css file. The panes should exist in your ascx file arleady. You can create as many panes as you want... just remember to have the "runat = server" in there or you won't see them when your'e ready to put a module in them. Then just put your HTML module in the appropriate panes and they should take up however much space you alotted in the skin.css.