Boldface letters are the extra-black ones — like these. Most word processors can produce these. They have only a few general uses.
First, they are used for chapter titles and section headings, exactly as is done in this document.
Second, they are used for the captions to illustrations, tables and graphs.
Third, they are sometimes used to provide very strong emphasis, as an alternative to italics. In this document I have used them in this way very frequently — probably too frequently:
The judicious use of boldface can provide variety and make a page more attractive to the eye, but it is never essential. If you can't produce boldface, use ordinary Roman type for italics for emphasis and important terms. If you do use boldface, don't overdo it.