DataNode:
A DataNode stores data in the Hadoop File System. A functional file system has more than one DataNode, with the data replicated across them.
NameNode:
The NameNode is the centrepiece of an HDFS file system. It keeps the directory of all files in the file system, and tracks where across the cluster the file data is kept. It does not store the data of these file itself.
Jobtracker:
The Jobtracker is the service within hadoop that farms out MapReduce to specific nodes in the cluster, ideally the nodes that have the data, or atleast are in the same rack.
TaskTracker:
A TaskTracker is a node in the cluster that accepts tasks- Map, Reduce and Shuffle operatons – from a Job Tracker.
Secondary Namenode:
Secondary Namenode whole purpose is to have a checkpoint in HDFS. It is just a helper node for namenode.
Referrence: http://doctuts.readthedocs.org/en/latest/hadoop.html
Referrence: http://doctuts.readthedocs.org/en/latest/hadoop.html
