OpenShift = Docker + Kubernetes. OpenShift is an orchestration PaaS provided by Red Hat. It is infrastructure-agnostic; you could run it on top of EC2, for example: imagine deploying Docker containers to EC2 instances.
What do we need to run Magnolia? Tomcat, MySQL, Magnolia WAR file. What if I want to add or remove a public instance to a Magnolia install, say in a High Availability scenario? I will need some orchestration machine - this is where OpenShift comes in.
OpenShift uses ActiveMQ to communicate between the orchestration machine ('Broker') and the Nodes.
Cartridge
A Cartridge is an OpenShift extension. Think library include. We might build a Cartridge to a DB platform like MySQL. We install Cartridges into Gears to provide functionality to our apps.
Gear
A Gear is a Container. We organize N > 1 Gears into a Pod.
Pod
A Pod is the atomic unit for OpenShift. It is the smallest manageable unit. Pods live on Virtual Machines. Virtual Machines are also known as 'Nodes'.
Cluster
A Cluster is a collection of Virtual Machines.
Links
There's a basic Ansible Role for installing OpenShift here.