Sections & Trees
The Umbraco back office consists of sections (sometimes referred to as applications) which contain trees. 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...
Both sections and trees can be created to extend Umbraco.
Describes Umbraco Sections, configuration and APIs
Describes Umbraco Sections, configuration, APIs and events