Composer is very useful tool or better dependency manager for PHP. Matthieu Moquet wrote excellent blog post about 5 features to know, and I am sure there is more about Composer on his blog posts.
Here is summary of the command line examples, learn to know, what they do:
$ composer update foo/bar $ composer update nothing $ composer update --lock $ composer require "foo/bar:1.0.0" $ composer init --require=foo/bar:1.0.0 -n $ composer create-project doctrine/orm path 2.2.0 $ composer init --require="twig/twig:1.*" -n --profile $ composer update symfony/yaml --prefer-source $ composer status -v $ composer update $ composer dump-autoload --optimize