A little history may help here. Docker Inc started as a platform as a Service company, dotCloud. In its core, they leveraged Linux Containers and to help them create, manage these containers; they built an internal tool, docker.
Now consider a couple of questions in your organization
How do you build, ship and run applications?
Can you do it in a repeatable way?
Is it an automated process?
Are you sharing code or processes from development to operation?
Do you ever encounter "Work on my machine"?
Yes! The docker is trying to answer this question.
Then What is the docker?
Enterprise Container Platform for high-velocity innovation Securely build, share and run any application, anywhere