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 Trees, configuration, APIs and events
Explains how to customise the backoffice search of a Section Tree.