Hosts and clusters, including datastore clusters, are providers of physical resources.
For hosts, available resources are the host’s hardware specification, minus the resources used by the virtualization software.
A cluster is a group of hosts. You can create a cluster using vSphere Client, and add multiple hosts to the cluster. vCenter Server manages these hosts’ resources jointly: the cluster owns all of the CPU and memory of all hosts. You can enable the cluster for joint load balancing or failover. See Creating a DRS Cluster for more information.
A datastore cluster is a group of datastores. Like DRS clusters, you can create a datastore cluster using the vSphere Client, and add multiple datastores to the cluster. vCenter Server manages the datastore resources jointly. You can enable Storage DRS to balance I/O load and space utilization. See Creating a Datastore Cluster.