An outline is a tool used to organize written ideas about a topic in a logical order. There are two main types of outlines - sentence outlines and topic outlines. A sentence outline presents the thesis statement, major topics, subtopics, and supporting details in sentence form using formatting like Roman numerals and capital letters. A topic outline presents the same elements in the form of words and phrases. An effective outline includes a thesis statement, major topics, subtopics, and supporting details. Outlining helps to show the relationship between ideas, simplify writing, and effectively organize one's thoughts.