Package an app once
An app may work on a developer’s computer but fail on a server because it needs different supporting software. A container image packages the app with much of that software. Starting the image creates a container: a running copy of the app.
Teams can test an image and then use the same one to run the live app, reducing surprises caused by different setups.
Where Docker fits
Docker provides tools to make and run containers; other tools can do this too. Containers on the same machine share the operating system’s core; a virtual machine has its own. Teams still manage data, security, updates, and connections to other services.