On WP Glossary, for instance, the footer includes the name of the website, a menu with links to different pages on the site, information about the license under which the content is made available and quotes from the lyrics for Hello Dolly.
The name of a site, a menu and copyright information are the most frequently seen elements in website footers. WordPress themes often include one or multiple widget areas in the footer as well. If your theme has such widget areas, you can customize them in Appearance → Widgets.