Word Styles
Styles can save a ridiculous amount of time that you would otherwise spend formatting with text. And many Word features rely on styles. You can't create a table of contents or use the document map unless each heading in your document has been assigned a heading style. Nor can you take advantage of Outline view and the commands for moving text around in that view. You can't cross-reference headings or number the headings in a document.
If you want to be stylish, at least where Word is concerned, you have to know about styles.
