I gave some of my thoughts on the topic here: http://stackoverflow.com/questions/17989306/what-does-docker...

Linux containers are its main ingredient. Docker itself provides a system for constructing the container images (with a neat super-fast caching/inheritance system that can pull from a central image repository), and manages the lifecycle of the containers. It also does some magic with ports and service discovery.

Technically - Creation, management, communication, networking. Smartly.

Ecosystem - more and more people every day are integrating with their tools, building business around Docker, or extending support of existing business to Docker.