Docker is an open-source tool that allows developers to package applications into containers to ensure consistency between development and production. It provides standardized packaging that isolates applications from each other and shares the same operating system kernel. Docker benefits developers by allowing applications to be built once and run anywhere without dependencies, and benefits DevOps by increasing efficiency and speed of the development lifecycle. The document then discusses who uses Docker, how to install it, the Docker engine architecture, images, registries, commands, and demonstrates Docker.