jQuery is a JavaScript library which is a free, open-source software designed to simplify HTML DOM tree traversal and manipulation, as well as event handling, CSS animation, and Ajax.
Twenty Fifteen is the default WordPress theme for 2015.
WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture and a template system.
YouTube is a video sharing service where users can create their own profile, upload videos, watch, like and comment on other videos.