Docker.com defines Docker as an open platform for developers and sys admins to build, ship, and run distributed applications.
This is what makes Docker so powerful. It provides most of the isolation benefits of traditional virtual machines without the overhead of running a guest operating system and a hypervisor. This allows you to have software applications that are packaged in Docker containers that you can be confident will behave the same way in development and production but are still lightweight enough to move around easily and to start and stop quickly.