The Document Tree

Essentially, the document tree is a document structured (in main memory) as a tree (data type). The nodes of the tree are objects (in the OOP sense) of type HTMLElement.

The basic interface, from which all the HTML elements' interfaces inherit, and which must be used by elements that have no additional requirements, is the HTMLElement interface.
HTML Living Standard ยง3.2.2 Elements in the DOM

The parent-child relationships of the tree represent the relationship contains of the serialized, UTF-8 representation of the document. For example, in the serialized document, the html element directly contains the head element. And in the document tree, the html element object is the direct parent of the body element object. Further, the order that siblings occur in the tree represent the order that element occur in the serialized document. For example, in an HTML document, the html element contains a head element immediately followed by a body element. And in the document tree, the head element would be the first sibling of html and body would be the second sibling.

Syntactic definition of an HTML element in the serialized document.