The Asset Tree is the primary way of getting around the WCM. It works just like the Dreamweaver or Windows Explorer tree, with the ability to expand folders by clicking the + next to a folder name.
To select a page, navigate to its location in the tree and click it.
When you click on a page (see 1), you interact with it using Tabs (2) and Panes (3). When working with a page, the most common situation is to Edit the page using the "Content" pane.