Still reading from "Don't Make Me Think." The chapter I'm reading now, called "Street Signs and Breadcrumbs," discusses some of the conventions that websites use to improve usability.
One convention is the use of persistent navigation to tell the user where he (or she) is in the website. There are 5 things that belong in every site's persistent navigation:
1. Site ID
2. A way home
3. A way to search
See the home page from Flickr--shows each of these elements in practice.
Sections are the primary navigation of the site. The top level of the site's hierarchy.