Twig's official page is http://twig.sensiolabs.org/
Pages, what if found most usable are these:
You can find introduction to Twig on Symofny's page too
and here: http://symfony.com/doc/2.0/cookbook/templating/index.html
and here: http://symfony.com/doc/2.0/components/templating.html
You can see default Twig's configuration in Symofny 2 here:
I think all Twig and Symfony has great official documentation and there is no need to rewrite anything about Twig here in this blog, that's why I'm just giving links to great sources. To know where to look is as same as good as to have all documentation here.