Outliner

Textual information is contained in discrete sections called "nodes", which are arranged according to their topic–subtopic (parent–child) relationships, like the members of a family tree.

Outliners are used for storing and retrieving textual information, with terms, phrases, sentences, or paragraphs attached to a tree.

In other words, as a hand-written work an outline is a writing tool, but on a computer, it is a general purpose format supported by a robust development and display medium capable of handling knowledge from its creation to its end usage.

Outliners are ideal for managing lists, organizing facts and ideas, and for writing computer programs.

There are two basic types of outliners: one-pane or intrinsic, and two-pane or extrinsic, each with its strengths and weaknesses.

The drawback is that, because the structure is not always visible, there is not as strong an overview of the whole or ability to quickly navigate between sections as with a two-pane outliner.

This view is similar to many file browsers and email programs (which can be thought of as three-pane outliners).