Instead of referring to an external CSS file via
<link rel="stylesheet" type="text/css" href="style.css" />
you can also add
<style type="text/css">
body { ... }
...
</style>
to the <head> section of your document. The main set back is that if you do that, you have to edit every single page where you want certain styles to appear instead of referring to one single document.
Internal CSS is a way to include CSS styles directly within the HTML document, using the