Twig is template framework for PHP, see http://twig.sensiolabs.org/doc/api.html. It has a very concise syntax, which make templates more readable.

Twig has the ability to cache loaded templates, which makes the rendering faster. This desired speed-up is enabled in the master branch. However, the mechanism requires to remove the cached files after changing templates. The command make empty-cache will do so. For development, one can disable the caching mechanism by changing TWIG_CACHE variable in Makefile.conf to TWIG_CACHE = <parameter key=\"cache\" type=\"constant\">false</parameter>. This will change the value for key "twig.environment.config" in the generated file src/lib/php/service.xml.

With Twig_Extensions_Extension_I18n, variables in templates are translated by the trans filter.