From Jason Kottke in a post on stop design’s site:

…semantics with regard to web design means that if you have a paragraph, surround it with

tags; if you’ve got a article title (or something), use an

tag; and if you have a list of items, use
    1. tags instead of separating them with breaks or using
      s. As much as possible, the tags surrounding the content of a document should describe what that content is and/or what it’s for.