As of writing this post Drupal 8/9 were in stable.

Bartik theme - Default theme in drupal.

• Bartik is the default user-facing theme, or the theme you first see when you install Drupal using the standard profile.
• Plain and enough for basic sites.

Seven theme - Default admin theme

• Default admin theme baked into the Drupal core.
• Looks quite plain and old. :)

Claro theme - An experimental admin theme.

• This is an administrator theme built into the Drupal core.
• Looks like some inspiration from a mix of Material and Flat UI's

Stark theme - A classic representation of Mark 1 by Tony Stark. ;) Only for demonstrative purpose.

• The Stark theme is provided for demonstration purposes; it uses Drupal’s default HTML markup and CSS styles. It can be used as a troubleshooting tool to determine whether module-related CSS and JavaScript are interfering with a more complex theme, and can be used by designers interested in studying Drupal’s default markup without the interference of changes commonly made by more complex themes.
• It's probably unsurprising to learn that the Stark theme provides the absolute minimal amount of markup and CSS. It's primarily for demonstration purposes, or for sites that have very strict markup and semantic requirements. Unless you plan to implement a custom version of each of Drupal's default templates, Stark probably isn't a good choice for a base theme. But it can be incredibly helpful in tracking down module-related markup and CSS bugs.

Classy theme - A subtheme of Stable and base for Bartik and Seven.

• Classy is a subtheme of Stable that adds many helpful classes that make it easier to quickly achieve a modern look with Drupal.
• Classy provides the rich markup familiar to users of previous versions of Drupal. Its markup provides many classes as well as an attempt at basic styling of core's markup. Classy is used as the base theme for both the Bartik and Seven themes. Its markup attempts to provide defaults that will be helpful to the majority of sites.

Stable theme - Base theme for many different themes

• Stable theme aggregates all of the CSS from Drupal core into a single theme. Stable theme is designed to be a fence for backward compatibility. The Stable theme markup and CSS will not change so any sub-theme of Stable will know that updates will not cause it to break.
• Stable is Drupal's "clean" base theme. It provides minimal markup and a small handful of CSS classes. The intent is that this Stable base theme will be used as a backwards compatibility layer protecting your theme against changes to core's markup and styling. If you're looking to build a custom theme with complete control over all of the markup and styling you should take a look at using Stable as your base theme.

Resources:

• Drupal Core themes
• Drupal Base Themes: Stable and Classy