Subsites
Give a section of your site its own layout, navigation, and logo, while it stays part of the same site and shares the same content.
A festival strand, an education programme, or a venue within a venue often needs to feel like its own place without becoming a separate website to maintain. Subsites do that. Under Website you pick a homepage and a layout, and the subsite is that page plus everything beneath it. Pages inside it inherit the layout automatically, and a page's template or an explicit layout choice still wins when you want an exception.
Its own navigation and logo
A header inside a subsite can root its menu at the subsite's homepage, so visitors see that section's pages rather than the whole site, and the logo and Home link point back to the subsite homepage. Headers and footers can also carry their own logo, so the subsite can be branded on its own while the main site keeps the site logo. The subsite's homepage can be hidden from the main site's navigation without affecting the subsite's own menu.
Same content, same preview
Subsites share events, people, venues, media, and everything else with the rest of the site; only the presentation changes. Preview shows the subsite's layout exactly as it will publish, including on preview domains.