HTML documents consist of a tree of tags that reveals the document's structure. The root of the tree is the HTML tag. The two largest branches of the tree are HEAD and BODY. Offshoots of HEAD include TITLE, STYLE, SCRIPT, ISINDEX, BASE, META, and LINK. Offshoots of BODY include headings (H1, H2, and so on), block-level elements (P, DIV, FORM, and so on), text-level elements (FONT, BR, IMG, and so on), and the ADDRESS element. Leaves on the offshoots include attributes such as WIDTH, HEIGHT, ALT, and HREF.
A document object model, or DOM, is also a tree that discloses the document's structure. The DOM, however, reports this structure in terms of objects and properties, rather than in terms of tags and attributes.
The root of the DOM tree is the document itself, the HTML object is the trunk, and the rest of the objects in the document branch from the HTML object as the HTML tags and attributes do.