Docker is used to create containers (packaged softwares) on operating system virtualization. Dockers are different from virtual machine in the sense that Virtual machines have thier own operating system. Docker containers are lightweight. Docker commands talk to docker daemon process which manages containers. Main classes in docker are Images, Containers and Services. We create containers using Images. Services can be used to scale the containers to work in a Swarm. Dockers images are stored in a repository called as docker registry. Docker Hub is a well know registry.