This document discusses Docker, an open source project that automates the deployment of applications inside software containers. It begins by describing common problems in application deployment and how virtual machines address some issues but introduce overhead. It then summarizes the history and rapid growth of Docker since its launch in 2013. The rest of the document dives into technical aspects of Docker like how images and containers work, comparisons to virtual machines, security considerations, the Docker workflow, and how Docker relates to DevOps and continuous delivery practices.