Docker is one of the most promising technologies these days. It is a container technology based on Linux. A very lightweight form of virtualization. Docker containers can be as small as 50 MB. Much smaller than traditional VMs.
The Docker project was started in March 2013 by dotCloud. In the mean while the makers of dotCloud sold dotCloud to CloudControl and raised $55 Million to focus only on the Docker development.
Check out my slides to “Docker Introduction”. I did this talk at the Webmontag in Mannheim. Feedback was very good.
I’m using Docker since beginning of 2014 in production and I love it. It’s a great technology!
Robert Reiz 
I use Docker at work in the context of micro API is very convenient to host numerous apis on the same machine.