Cluster is said to be collection of queue manager's logically linked together to balance the load on the remote machine (i.e if it is remote queue managers which processes your message). And there are two kinds of grouping on the queue managers.
1) Full repository
2) Partial repository.
The full repository queue manager knows all other queue managers in the cluster and when a message comes to it, and if that is not capable to process the message, it will search a queue in other queue manager in the cluster and ask it to process the message.