New libraries, frameworks, engineering practices and tools have dramatically changed the creation of PHP software over the last 3 years. Composer played a key role in facilitating the rebirth of the programming language the web is built on. Drupal 8 embraces this new style of PHP development by replacing some of its core subsystems with 3rd party components and libraries, managed with Composer.
By Nils Adermann (http://naderman.de/)