There are three different ways you can add stylesheets to your web pages. You can import them, declare them within the page, or declare them inline - or within the tag itself. When you import a stylesheet, the only requirement is that the file have the css extension. When you declare styles on a page, they are contained within the style tag, with the modifier type="text/css". The modifier isn't absolutely necessary, but I am fairly sure that certain things may require it in order to work right. And finally, declaring them inline requires nothing more than the modifier style="keyword:value;keyword:value". No brackets or tag names or classes required here; just the keyword:value pairs separated by semicolons. Here are examples of them:
<link rel=stylesheet href="css_url.css" type="text/css">
<style type="text/css">
p{keyword:value}
</style>
<p style="keyword:value;keyword:value">
The first one is importing a stylesheet using the link tag. I don't know what the rel=stylesheet part does, but I assume it's telling the browser that it's importing a stylesheet. The href is the location of the stylesheet, and the type="text/css" modifier is in there too. Next is the declaration of a style within the page itself. These belong in the header (head tags). Other than that it works the same way. Lastly, there's the inline declaration. As you can see, all there is is the style modifier with any keyword:value pairs contained in quotes.
And there you have it! All that's left to learn now is what those keywords and values can be!