How an application is packaged together with everything it needs to run the same way in every environment.
Container packages application source code together with libraries and configuration into a runnable unit, sharing the operating system kernel with the server rather than carrying an entire operating system like a Virtual machine.
Because of this, containers start in seconds rather than minutes, and a server can run far more containers than an equivalent number of virtual machines.
Note on data: containers are designed to be deleted and recreated at any time, so any data that needs to be retained must reside in a volume or external storage service, not within the container itself.
