Let's define this line as our first page, or the trunk of the tree...

and this as a branch, or the hyperlink we take from the first page

so on and so forth...

The red circle indicates the new page the user jumps to with the "trunk" as the initial page

as you can see there was another option we could have taken, another hyperlink we could have clicked.

As we move down a branch, and further away from the trunk, we can see the options not taken, links not clicked, and all the links off of those, forming an entire tree for us to explore and move about.

This is shown by the other branch

We can move back and forth along branches, clicking links or pressing the back button, but cannot jump to the other side of the tree.