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, such as sqndr's excellent Drupal 8 theming guide.
PHPTemplate has been discontinued for theming in Drupal 8; the Twig markup language is used instead. This will make Drupal 8 themes much more secure because PHP calls will no longer exist in theme files. This will also make theming easier to understand for non-programmers.
For a collection of useful materials for themers, see Theming and Front End Development with Drupal.