1. Usability improvements: improved responsive behavior for all types of devices, clean new course editing interfaces, annotation of PDF assignment submissions, improved course management, a more user-friendly text editor, additional name fields, a new course format that lets you make a course out of any single activity, support for non-Gregorian calendars and much more.
2. Performance improvements: new options for handling sessions using backends like memcached, full support for PHP OPcache, faster admin menus, improvements on MS SQL and Maria DB and more.

For more details on these see the official Moodle 2.6 release notes, or browse our Moodle 2.6 New Features page which has nice screenshots.

Something new for this release is a series of videos we’ve made to show some of the main new features – we hope you enjoy them!