For those unfamiliar, Docker containers are virtual linux containers that wrap up a piece of software as isolated and self-contained systems that include code, runtime, tools, libraries and file storage. They are lighter weight than a virtual machine, allowing for more density in packing hundreds of containers and maximizing the CPUs available.
The service provides support for Docker containers on demand as well as support for container orchestration tools such as Apache Mesos or Docker Swarm.
ACS clusters are composed of masters and agents. Masters provide container orchestration and deployment management. Agents provide the computing power for your workload. A single cluster must include a minimum of three virtual machines: one master, one public agent and one private agent. For high availability you are recommended to deploy either three or five masters to your ACS cluster. Masters always use D2-size virtual machines, but for agents you can select any size VM. The prices for virtual machine instance sizes can be found here.