There is three ways to specify CSS rules in HTML:
<head>
<link rel="stylesheet" type="text/css" href="mystyle.css">
</head>
see HTML - The link element (inter-document relationships)
<style>
a {color:blue;}
</style>
HTML - Style (Element|Embedded Style) - Attribute
With inline Css, you use the style attribute to define the style.
<p style="color:blue;">This is a paragraph with an inline Css style.</p>