Can someone please explain what is Docker exactly? I've read this page, http://www.docker.com/about_docker/docker/, and didn't understand. What exactly does it encapsulate? What does the application see when it queries its environment?
What does it add over linux containers?
Ecosystem - more and more people every day are integrating with their tools, building business around Docker, or extending support of existing business to Docker.
In general, Docker encapsulates and runs your processes and file systems in an exactly repeatable way, and in a format that you can replicate across a wide variety of machines.