CSS is an acronym for Cascading Style Sheet, which contains code specifically controlling the visual style of your site. In relation to WordPress, CSS is spread over several documents, each serving a specific purpose. For example:
reset.css – contains special code for resetting specific styling instructions coming from the browser, previous page loads, or plugins that can break the page layout.
style.css – your theme’s main stylesheet containing all of the instructions for the theme’s layout, colors, fonts, borders and graphical elements such as icons and backgrounds
typography.css – a stylsheet containing just font and text styling
CSS does not control the actual structure or order in which elements appear. That is determined by the theme’s markup (combination of HTML and PHP code) in the templates.
To learn more about CSS, see the following resources: