This guide is about creating themes for Drupal 8.
Quite significant changes were introduced in the theme system between Drupal 8 and 7. This guide contains information that applies to Drupal 8 and refers sometimes to Drupal 7 in order to explain new concepts and approaches.
Outside drupal.org other theming guides can be found, like for example sqndr's excellent Drupal 8 theming guide.
For a collection of useful materials for themers, see Theming and Front End Development with Drupal.