A tag is a label for a set of data. Dependent on the context of use and the markup language, a tag serves to highlight, classify or structure textual elements. Also known as an HTML-tag, it signifies a specific domain with an HTML document, which can then be displayed in various different ways for various purposes. H1, H2 and H3 tags should guide the reader through the site, with concise text informing them quickly and efficiently of site content. The H1 tag is the most important, top level heading. The clue is in the name - an H1 tag should only be used once.