You have in the head of your HTML
<link rel="stylesheet" type="text/css" href="site.css" />
and an external file in a location that is determined by the href=" ... " portion, the above example points to a stylesheet called site.css in the same directory as the HTML page that has called it.
Your stylesheet is a file named by what you want to refer to it by and has the extension .css but in reality the extension could be anything you like but more relevent to keep use of the .css extension because editors and software that is used for production of web sites know then that its a stylesheet.
Unlice CSS in the HTML page, your stylesheet does not need the HTML tags for stylesheets.