Bartik was introduced in Drupal 7 as a new, clean and simple theme. The theme has some new cool features in Drupal 8 and is also completely responsive. Bartik is the default user-facing theme in Drupal 8.
base theme: classy
Moving Bartik forward
During the development of Drupal 8, the Bartik theme was barely making any progress. For this reason, the maintainership for the theme has changed. Emma Karayiannis is the new Bartik maintainer. This happened after the first couple of betas. Since then, Bartik really started to move forward again.