external stylesheet
<html>
<head>
<link rel="stylesheet" type="text/css" href="http://www.yoursite.com/style.css">
</head>
<body>
bla bla bla
</body>
</html>
with style.css containing amid the other styles
a:hover { background-color: blue; color: white; }
header styles
<html>
<head>
<style type='text/css'>
<!--
a:hover { background-color: blue; color: white; }
-->
<style>
</head>
<body>
bla bla bla
</body></html>
inline styles
<html>
<head>
</head>
<body>
<a href='thisfile' style="hover { background-color: blue; color: white; }" > thisfile </a>
</body>
</html>
most use external stylesheets
only one file need be altered to update the whole site
header styles, contain the full stylesheet in every page
inline styles contain the full style in every link on every page
they get exponentially bigger
each has a purpose
external stylesheet to style a site
header style to make a single page different
inline style to make a single element different