This document discusses Docker, including what it is, why it is used, and how it works. Docker provides lightweight software containers that package code and its dependencies so the application runs quickly and consistently on any computing infrastructure. It allows applications to be easily deployed and migrated across computing environments. The document outlines how Docker addresses issues like managing multiple software stacks and hardware environments by creating portable containers that can be run anywhere without reconfiguration. Examples of using Docker for microservices, DevOps, and data centers are also provided.