Overview
elementary OS is a Linux distribution based on Ubuntu LTS. It promotes itself as a "thoughtful, capable, and ethical" replacement to macOS and Windows and has a pay-what-you-want model. The operating system, the desktop environment (called Pantheon), and accompanying applications are developed and maintained by Elementary, Inc.
Design philosophy
The human interface guidelines of the elementary OS project focus on immediate usability with a gentle learning curve, rather than full-fledged customization.
The three core rules the developers set for themselves were "concision", "accessible configuration" and "minimal documentation".
Since its inception, elementary OS has received praise and criticism for its design. Wired claimed that it closely resembled macOS, visually and in user experience. The elementary developers maintain that any similarities are unintentional.
Pantheon's main shell is deeply integrated with other elementary OS applications, like Plank (a dock), Web (the default web browser based on GNOME Web) and Code (a simple text editor). This distribution uses Gala as its window manager, which is based on Mutter.
From Wikipedia (CC BY-SA 4.0).