Hypertext markup language (HTML) is a method used to create structured documents that can be viewed on the World Wide Web using a browser.
A structured document is one that contains identifiable elements, such as body paragraphs and headings. A document is visually structured when the elements have different looks (for example, the headings are in a different weight and type of font than the body text). A document is truly structured when the different elements have unique names, and you apply a structure to a string of text to identify it (to mark it up). Once you've marked up a document, viewers interpret the markup to determine how to display the document.
For example, if your document has body paragraphs and headings, a particular viewer may display all headings in 12-point Helvetica bold font, and all body paragraphs in 10-point regular Times. A different viewer may display the elements completely differently. However, each viewer displays the elements consistently. That is, in a given viewer, all level one headings look identical.