Elementary OS is a Ubuntu-based Linux distribution for desktop computers with unique UI design and in-house applications. The main goal of the project is to offer a complete user experience from ground up.
History and Introduction
Elementary started out as a GTK2 theme and icon set created by DanRabbit. As the popularity grew, elementary was going beyond being a theme and a group of developers and designers started working on Elementary OS 1.0 Jupiter, which was initially released April 2011 (using Ubuntu 10.10 as its base). Jupiter used Gnome 2.32 as its desktop environment but added a custom theme and icon set, Docky dock and few in-house application like Dexter contacts and Postler email client. They also made modifications to Nautilus file manager.
Luna, the second iteration of Elementary OS, is based on Ubuntu 12.04 LTS release and inherits most of its underlying code and sometimes its problems as well. Most of the visible UI and applications has been developed by elementary team themselves.