Docker is an open source project that helps automate the application deployment process to application containers.

Docker containers package application software within a comprehensive file system that comprises anything and everything that the application would need to run, including the application code, runtime environment, system tools and libraries that you usually request to carry out an installation, on your application server during deployment. With this feature, you ensure the application will execute and behave like any of your other environments, irrespective of the environment it is currently being executed in.

Docker offers an additional feature along with the ability to abstract and automate an OS simulation, both on Windows and Linux operating systems. With the help of the resource isolation feature of Linux--a kernel-like kernel namespace--and cgroups--a union capable file system such as OverlayFS--Docker provides the feature of running independent containers inside one Linux system without having to start and maintain virtual machines.

This ability of Docker is especially helpful for setting up highly distributed systems. Creating and managing containers makes it easy to establish a machine or multiple virtual systems with running multiple applications, scheduled tasks, and any additional processes. Also, the deployment activities of infra teams become easier with Docker instances prepared to be readily deployable to any platform with PaaS-style of deployment.

The following diagram represents Docker's motivation to build, ship, and run distributed applications anywhere: