The Umbraco backoffice consists of sections (sometimes referred to as applications) which contain Trees.
Each section is identified by its name in the top navigation ribbon of the Umbraco Backoffice.
For example, when you load the backoffice, you'll see that the 'Content' section contains one tree: the content tree. Meanwhile, the 'Settings' section contains a number of trees such as Stylesheets, Document Types, Media Types, etc...
You can create your own sections and trees to extend Umbraco.