Sections & Trees
The Umbraco back office 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 back office 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