What is Docker?

Docker packages an application and all its dependencies into a container — a lightweight, portable unit that runs identically on any system. Unlike virtual machines, containers share the host OS kernel, making them fast to start (seconds vs minutes) and efficient with resources. Key concepts: Dockerfile (build instructions), Image (built template), Container (running instance), Docker Compose (multi-container apps). Docker solves "it works on my machine" by ensuring identical environments from development to production.