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<h1>is used to markup the main topic of your page</h1>
<h2>is the markup for the "second level" subtopics on the same page</h2>
The concept is a lot like an outline of the original document -- in fact, that is the idea behind HTML. You begin with a document of some kind and then "mark it up" for the web. You may find a good orientation to this in the following thread: I Wish Someone Had Told Me When I Started [webmasterworld.com]