DC/OS (Datacenter Operating System) is a distributed operating system based on the Apache Mesos distributed systems kernel. It allows users to run frameworks like Marathon and Spark across AWS, CentOS, and other platforms. Marathon is the distributed init system for the DC/OS cluster and acts as a production-grade container orchestration platform, providing high availability, support for multiple container runtimes, stateful applications, and other features. The document discusses getting started with DC/OS using Vagrant and Marathon and provides demo links for Marathon and Jenkins.