Twig for Template Designers
– The _Twig for Template Designers_ page is fairly technical at times, but it's about the most accessible overview you're going to get about Twig from the official docs and a worthwhile reference.
Twig Tag Reference
– All native Twig supported tags, filters, functions, tests, and operators.
Full Templating Reference
– In addition to the native Twig tags, Craft adds support for several more. The Craft Docs go into detail on all of the tags and variables Craft makes available to you in your templates and give several examples of building common pages.