There are six html heading elements or tags. From h1 to h6.

<!--html-->
<h1>Heading 1</h1>
<h2>Heading 2</h2>
<h3>Heading 3</h3>
<h4>Heading 4</h4>
<h5>Heading 5</h5>
<h6>Heading 6</h6>

These elements define the headings of documents hence, they are known as heading tags.

Use these elements for web document headings. These tags are in order of importance, the most important heading should be defined with h1 and the next most important heading should be with an h2 tag and so on.

Html validation and for seo purposes heading tags should not be used for text stress such as to emphasis the size of some text. Heading tags are for web document headings and most web crawlers do crawl heading tags.