Hadoop has 5 daemons.They are NameNode, DataNode, Secondary NameNode, JobTracker and TaskTracker.
NameNode It stores the Meta Data about the data that are stored in DataNodes.
DataNode It stores the actual Data.
Secondary NameNodeIt is the backup of namenode and it just contains entire metadata of data nodes like data node address, properties and block report of each data node.
JobTrackerJob Tracker is a master which creates and runs the job. JobTracker that runs on name node, allocates the job to TaskTrackers.
TaskTrackerTaskTracker is a slave and runs on data node. TaskTracker runs the tasks and reports the status of task to JobTracker.