From http://www.w3schools.com (Copyright Refsnes Data)

HTML DOM Node Tree

The HTML DOM views a HTML document as a node-tree.

All the nodes in a node tree have relationships to each other.

The HTML DOM Node Tree (Document Tree)

The HTML DOM views a HTML document as a tree-structure. The tree structure is called a node-tree.

All nodes can be accessed through the tree. Their contents can be modified or deleted, and new elements can be created.

The node tree below shows the set of nodes, and the connections between them. The tree starts at the root node and branches out to the text nodes at the lowest level of the tree:

DOM HTML tree

Node Parents, Children, and Siblings

The nodes in the node tree have a hierarchical relationship to each other.

The terms parent, child, and sibling are used to describe the relationships. Parent nodes have children. Children on the same level are called siblings (brothers or sisters).

In a node tree, the top node is called the root
Every node, except the root, has exactly one parent node
A node can have any number of children
A leaf is a node with no children
Siblings are nodes with the same parent

Look at the following HTML fragment:

<html>
  <head>
    <title>DOM Tutorial</title> 
  </head>
  <body> 
    <h1>DOM Lesson one</h1> 
    <p>Hello world!</p> 
  </body> 
</html>

In the HTML above, every node except for the document node has a parent node:

The <html> node has no parent node; the root node
The parent node of the <head> and <body> nodes is the <html> node
The parent node of the "Hello world!" text node is the <p> node

Most element nodes have child nodes:

The <html> node has two child nodes; <head> and <body>
The <head> node has one child node; the <title> node
The <title> node also has one child node; the text node "DOM Tutorial"
The <h1> and <p> nodes are siblings, and both child nodes of <body>