My basic understanding regarding docker and containers.

What is Docker?
Docker is a container management system. Each container run in an isolated manner. each container is responsible for its assigned application to run so docker enables user to build, deploy, run containers so using single OS image.
Why docker?
Cost effective, easy to manage, no worries to purchase different OS license.
What is container?
Container is like an instance of an OS on which only limited things are installed which are necessary to run an application for which container is instantiated. They provide the similar functionality like of a VM but with virtualize OS.
Docker workflow?
USer>>docker client>>docker host>>docker registry>>docker image
How container created?
docker helps in creating container with the help of kernel on which docker is installed.