" What is cluster, single node cluster and node?"

[#1]

What do you mean by cluster, single node cluster, and node?

[#2]

The cluster consists of one or more computers/machines working together to provide high availability, reliability, and scalability towards service being provided to clients. If one server/machine fails then work/resources get distributed among other machines in the same cluster.

Single node cluster or pseudo-distributed cluster is the one in which all daemon like NameNode, data node, Jobtracker, and tasktracker runs on the single machine. Default replication factor is 1.
Multinode cluster is a cluster which is basically used in master-slave fashion where master and slaves runs on different machines and master node/machine runs Namenode and TaskTracker daemons and slave machine runs Datanode and JobTracker daemons.

Note: - YARN is a cluster resource management.

[#3]

TaskTracker & JobTracker doesn't exist with YARN. The default replication factor is 3.