A Docker container is a lightweight, stand-alone, executable package of software that includes everything needed to run an application: code, runtime, system tools, system libraries, and settings. It packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another.
For example, a Docker container could contain a web server, a database, and an application server, all running on the same host system. This means that the application can be deployed and run quickly and reliably on any environment, regardless of the operating system.