Azure Service Fabric is one of the PaaS services offered by Azure. It is based on the notion of containers and services: unlike Compute services (Web Roles and Worker roles), your code does not run inside one (or more) Virtual Machines, but instead they are run inside a container, sharing it with other services.
It is important to note that here, and throughout Microsoft articles and documentation on Service Fabric, container is intended in its more general meaning, not as a "Docker" style container.
You can have (and you'll typically have) more than one container, which will form a cluster. Services running inside the container can be, in increasing order of "awareness"