HTML has six levels of headers that are labeled 1 to 6 with 1 being the largest and 6 the smallest. Normally the first heading in a document is tagged <h1> or <h2>.
<h1>This is a level 1 Header</h1>
<h2>This is a level 2 Header</h2>
<h3>This is a level 3 Header</h3>
<h4>This is a level 4 Header</h4>
<h5>This is a level 5 Header</h5>
<h6>This is a level 6 Header</h6>
Note that the tags are not case sensitive. That is <H1> is the same as <h1>.