What is TaskTracker and what is does?

TaskTracker are slave nodes that do the actual computation related to a job in MapReduce part in Hadoop. It has a loop that sends heartbeat method calls to jobtracker every 5 seconds , indicating its existence and  status of its task. JobTracker assigns a new task when the old one is completed. Some TaskTracker performs map task and after completing the map task, the reduce task is started.The output of the reducer task is the final output which is submitted to the client by the JobTracker.TaskTracker has  fixed number of slots, for map task and reduce task.