Composer.json defines the required libraries for a project, and allows
projects to define metadata about themselves for package management systems
http://packagist.org/about-composer
When a project is set up, a composer file can be used to initialise the required dependencies. This can be more flexible than git submodules, as the dependency can 'float' to the latest stable version.
There are a wide variety of packages available to install, either through URLs
embedded in a project's composer.json or by searching the packagist website.
cd /path/to/project/;
# this directory should contain the composer.json file
wget http://getcomposer.org/composer.phar;
php composer.phar install;