Docker is a platform for developing, shipping, and running applications. It provides containers that package applications and dependencies together allowing them to run seamlessly on any infrastructure. The document discusses Docker concepts like containers, images, and the Docker ecosystem. It also provides examples of using Docker with various applications and frameworks like PHP, Java, .NET, Nginx, and Apache. Managing Docker containers at scale can be done with tools like Kubernetes, Docker Datacenter, Rancher, and Prometheus for monitoring.