What a Container Is

This topic explains what a container is: a program running on your own machine's kernel, with a private view of files, processes and network, started from a read-only image. That one idea explains why containers start in a fraction of a second, how they differ from virtual machines, and why removing a container never damages the image it came from. It covers the shared kernel, containers compared with virtual machines, and the difference between an image and a container. It leaves out how Linux builds the isolation (namespaces and cgroups), which you do not need in order to use Docker well.