CSS Stands for Cascading Style Sheets.
CSS goes in the "style" tag inside the "head" tag.
An example CSS rule is the background-image rule, which sets the background image of an element. For example, the following paragraph has the rule:
#example { background-image: url(cat.png); }
This paragraph has id="example" and gets the background image defined in the style tag.
You can also include an external css stylesheet by using the "link" tag, like so:
<link href="/style.css" rel="stylesheet" type="text/css" media="all">
Including external stylesheets is a great way to ensure a consistent style acress an entire site.