Sectioning commands provide the means to structure your text into units.
All sectioning commands take the same general form, i.e.,
\chapter[optional]{title}
In addition to providing the heading in the text, the mandatory argument of the sectioning command can appear in two other places:
1. the table of contents
2. the running head at the top of the page
You may not want the same thing to appear in these other two places as appears in the text heading. To handle this situation, the sectioning commands have an optional argument that provides the text for these other two purposes.
The sectioning commands have *-forms that print a title, but do not include a number and do not make an entry in the table of contents. For example, the *-form of the \subsection command could look like:
\subsection*{Example subsection}