Working with outline view
An outline view is very much like a table with the exceptional ability that allows you to expand or collapse record rows that contain hierarchical data. Unlike a table, items in an outline view are not in a flat list, but rather may be organized in a hierarchy, like files and folders on a hard drive, or insurance providers and policy information of a insurance policy.
To explain about outline view hierachy, Outline View Programming Topics for Cocoa could not have put it better. And hence this quote: "An item in an outline view is expandable if it can contain other items. An expandable item is distinguished visually by a disclosure triangle, which points to the right when the item is collapsed and points down when the item is expanded. Clicking on the disclosure triangle causes the item to be expanded or collapsed, depending on the new state of the triangle. An item can be expanded even if it contains no items. An Option-click on an item's disclosure triangle expands or collapses all of its contained items."
Refer to Working with tables for more information about column sorting, addition, deletion, dragging and commits.