The major categories of outlines are formal, informal and cluster outlines. The purpose of an outline is to help organize thoughts for a longer written or spoken work.
Teachers often require formal outlines using Roman numerals or alphanumeric designations for each section with a formal hierarchy, which makes it easy to tell the difference between headings, subheadings and other levels of importance. Informal outlines, which are more often used when an outline does not need to be turned in, consist of thoughts jotted down in any organized way that works for the composer. Cluster outlines are related to informal outlines in that they don't have any specific shape. To craft a cluster outline, the composer writes down a central idea and then clusters related ideas around that idea. This may be used to brainstorm for a more formal outline, or it may be used as a composition guideline.
Outlining is often described as a type of brainstorming, which consists of getting ideas down on paper in preparation for the composition of some longer work. It is in essence a road map for composition designed to get major ideas down so that it's easy to compose in an organized fashion. Major ideas are followed by lesser ideas that give detail or direction for later composition. Provided there is no requirement for a specific type of outline, the best outlining technique is whatever most helps composers organize and expand upon their thoughts.