Mesos provide the core resource management and scheduling, think of it as the Kernel of an (DC) operating system. On top of Mesos you usually need:
1- Container/Workload Scheduling capability which is Marathon, now built in and integrated in DC/OS so you are just up and running in minutes 2- Management (Beautiful DC/OS UI and CLI) to manage all cluster and workload operations. It also includes Installer (of all components including Zookeeper, networking, ..etc) 3- App Store (Universe) that allows you to easily find and deploy latest packages such as Kafka, Cassandra and Spark with a single command, in regular Mesos you needed to find the Framework that worked with a specific version of Mesos, troubleshooting, ...etc. Now it's an app store like experience 4- Core infrastructure components such as very cool service discovery and loadbalancing (Minuteman), Storage drivers, CNI support
If you want to run docker containers, DC/OS also gives you the ability to use the docker daemon or the new Universal containerize (technology preview) which we just released.
Because mesos is very modular and very powerful, it was built to have modular implementation of all system components, but our team and our founders ran some of the largest implementations of Mesos (Twitter, Airbnb, ..etc) and they complied all thes best practices and the bits and pieces in DC/OS.
Hope that explains,