The following are the primary take-aways from this "Web Building Blocks" section of What Makes the Web Work?.
We use HTML (hypertext markup languge) to identify parts of a site (e.g. headers, footers, navigation, paragraphs, list) as well as to add media (e.g. images, audio, video) and links (hyperlinks).
We use CSS (cascading style sheets) to style the various parts of your site. CSS is reponsible for enhancing the visual aspects of your site and can be as important in making your site effective as HTML.