The default Drupal frontend theme, this will be active for all content pages when Drupal is initially installed. It is in fact a subtheme of Classy—that is, it is based on or "built on top of" the Classy theme.

This is the default backend/admin theme. This can also be set as the default frontend theme, but it is really designed to be the common administration theme. This too is a subtheme of Classy.

This is a very minimal theme that includes the smallest amount of HTML markup and styling. Unlike Bartik and Seven, this theme is not built on top of anything and shows off the raw HTML markup that comes out of modules. As such, it's useful to developers to determine whether module-related CSS and JavaScript are interfering with a more complex theme.

Classy is hidden from the UI at Manage | Appearance because you are not supposed to use it directly; is designed as a theme on which to base other themes – it is a base theme.

Classy is so-named because it presents Drupal's common classes on HTML elements within the mark-up that render on a page.

Both the Bartik and Seven themes are based on Classy.

The Stable is another hidden theme that is used if you only want to see the stable Drupal core mark-up. Creating your own themes is beyond the scope of this module (for more on themes see Module 3, Drupal 8 Theming with Twig) but what we will say is that if you make a brand new theme and you don't explicitly tell Drupal that your new theme is based on Classy then the Stable theme is used by default. Themes based on Stable will present much leaner mark-up than those based on Classy.