HTML is a language for describing text using tags. Some tags like <HTML>, <BODY>, and </HTML> are required, while others like <BR> and <HR> are optional. Tags often come in pairs to delineate sections, and HTML documents typically contain tags that divide the document into a head and body section. The document discusses several common tags and their purposes, such as setting text formatting, creating paragraphs and tables, adding hyperlinks and images. It also notes that HTML documents may appear differently in different browsers since HTML only describes text and not layout.