Docker is a set of platform as a service (PaaS) products that use OS-level virtualization to deliver software in packages called containers. Containers are isolated from each other and bundle their own software, libraries and configuration files; they can communicate with each other through well-defined channels. Because all containers share the services in a single operating system kernel, they use fewer resources than virtual machines.
The service has both free and premium levels. The software that hosts the containers is called the Docker Engine. It was first started in 2013 and is developed by Docker, Inc.