What is a Docker container?

A Docker container is a small, portable, self-contained package that contains everything required to run a program, including the application code, runtime, system tools, libraries, and settings. Docker containers are built using Docker images, which are read-only templates that define the program and its dependencies. Each Docker container is separated from the underlying infrastructure and other containers, which means it has its own file system, network interfaces, and resource allocation. This isolation provides consistency and security, as containers can run on different environments without any modification.

In contrast to virtual machines (VMs), where virtualization takes place at the hardware level, containers virtualize at the app layer. They can use a single machine, share its kernel, and virtualize the operating system to execute separate processes.