An outliner is a computer program that displays a document in a tree-structured way, so that parts can be collapsed, giving a greater overview of the whole document.
One distinguishes between single-pane and two-pane outliners.
Single-pane outlining is builtin in such programs as Microsoft Word, and many popular programming editors, such as Emacs, which has an outline-mode.
Two-pane outlining is used by some PIM software. The document is divided into pages, each corresponsing to a node in a tree, which is shown in the left pane.
A popular two-pane outliner is TreePad. Outlining software is surprizingly identical. Other, similar outlining PIM databases are: Action Outline, Jot+ Notes, Skwyrul, MyInfo, Mybase, KeyNote - the last being OpenSource.