The Guide is a two-pane extrinsic outliner, similar to mindmapping in some ways, designed to help you organize information hierarchically.
The Guide evolved from the need to have an application that could organize information and ideas in a hierarchical, tree-like structure. Tree-based structures are frequently employed to manage information through a "divide-and-conquer" approach, wherein each level of the tree represents a further level of specialization of the parent-level topic — the best example of this being a book. The Guide is an application that allows you create documents ("guides") which inherently have a tree (which you can modify as you please) and text associated with each node of the tree. The text itself is of the rich-text variety, and the editor allows you to modify the style and formatting of the text (fonts, bold, italics etc).