A *nix command line question

hello forum and *nix users,

i recently bought and tried out a new PHP/MySQL book from sitepoint.
And i have a question…

Can someone please explain to me what the double dashes mean in the following command (–user=mysql), or for that matter, any command?

scripts/mysql_install_db --user=mysql

…it bothers me because the book doesn’t explain its significance
thanks guys!