A cluster is a set of instances, spanning across one or more nodes, all running identical configuration and offering an identical set of runtime services. Each cluster must include one server designated as the administration server. If you have more than one cluster, you can administer all clusters from a single master administration server. The master administration server retrieves the information about all the clusters and provides the interface for managing the servers installed in their respective clusters.
All the instances in a cluster are required to be homogeneous. For example they run on an identical operating system version (and patches) and service packs, run an identical web server configuration and offer identical services.