Headings form a part of the logical structure of an HTML document. There are six heading tags <h1> to <h6> each having its ending tag.
To get better ranking in search engines, make sure that the web page headings contain all (or the most important) keywords and key phrases. If the same keywords and key phrases are repeated in the paragraph following the heading, the page will certainly get a boost in ranking.
You should NEVER use headings to increase the size of text. You have at your disposal the <font> tag, and the <small>, <big> tags for that purpose. Furthermore, you can gain a better control over font size and other text properties using Cascading Style Sheets.
Since headings provide a logical structure to the document, another important thing to remember is NEVER to use smaller level headings before the higher. Thus, <h4> should not be used before <h2> (though, HTML does not enforce this).
