Cluster is a set of services running on the set of hosts. ADCM allows you to create a lot of clusters with different configurations and different service distributions.
A single cluster allows only a certain set of services. For example, if you create a Hadoop cluster using a Hadoop bundle, there will be no way to install services from an ADB cluster created using an ADB bundle.
You can see the list of clusters at the CLUSTERS tab.
CLUSTERS tab in ADCM