HTML headings are denfined by using <h1> to <h6> tags. The lower the number the more important the heading, with <h6> being least important.
Heading tags serve multiple purposes. Headings help create organization of a pages content. A heading 1 tag should be used for the most important element on the page such as the main title that describes what the page is about. Headings 1-6 can then be used to organize and idenitify sub content of the main heading.
Heading tags not only organize and show imprtance of content to human viewers of the page, but search engines rely on headings to inform what keywords are important to the page.
Only using a Heading tag once on a page. The <h1> denotes the most important keyword(s) on the page. Using it more then once will confuse what is most important.
Other heading tags can be used more then once. Use the tags to create a hierarchy to the content on the page.
A heading tags formatting can be modified through styles (CSS), such as color and size.