Sections & Trees
The Umbraco backoffice consists of sections (sometimes referred to as applications) which contain Trees.
Each section is identified by an icon in the left-hand side 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 whereas the 'Settings' section contains a number of trees: Stylesheets, Content Types, Media Types, etc...
You can create your own sections and trees to extend Umbraco.
Describes Umbraco Sections, configuration and APIs
Describes Umbraco Sections, configuration, APIs and events