Cascading Style Sheets, usually shortened to CSS, is a type of style language that WordPress theme developers can use to define how webpages are displayed. Developers combine CSS with HTML to instruct WordPress on how to display pages. All WordPress Themes have a file named style.css that includes all of the different style rules that theme uses.
WordPress users who are interested in learning CSS will find a number of different tutorials online. There are a number of CSS tutorials that are aimed at WordPress theme creators, making it easy for users to learn how CSS works with WordPress. Even without tutorials, users can often see exactly what CSS is doing to their WordPress site simply by viewing their style.css file.
One advantage of using CSS rather than HTML is that CSS allows designers to create different sections on the same WordPress page. In one section, the H1 tag might format text with a specific font and size. In the next section, H1 may be redefined to use a different font and size. This is done by using classes and identifiers to create different definitions of the same element. For example, a designer may want to use one definition of H1 for the page title, but another definition of H1 for a post title.