Job Tracker -
JobTracker is the service within Hadoop that runs MapReduce jobs on the cluster.
JobTracker process runs on a separate node and not usually on a DataNode.
JobTracker is an essential Daemon for MapReduce execution in MRv1. It is replaced by ResourceManager/ApplicationMaster in MRv2.
JobTracker receives the requests for MapReduce execution from the client.
JobTracker talks to the NameNode to determine the location of the data.