It refers to the structure of HTML document and their arrangement in a hierarchically order. These include the properties include content as well as attributes in relation to HTML DTD. These may also resemble headings / paragraphs / hypertext / links / lists / embedded media and others.
HTML elements are the elements which represent headings & paragraphs. It may also represent the links called as hypertext links, embedded media and also other structures.
HTML elements are used in HTML document to arrange the content and also it indicates the structure of an HTML document.