What is a Jobtracker and tasktracker in hadoop?

There is one JobTracker(is also a single point of failure) running on a master node and several

tasktracker running on slave nodes. Each tasktracker has multiple task-instances running and every task tracker reports to jobtracker in the form of heart beat at regular intervals which also carries message of the progress of the current job it is executing and idle if it has finished executing. 

Jobtracker schedules jobs and takes care of failed ones by re-executing them on some other nodes. In Mrv2 efforts are made to have high availability for Jobtracker, which would definitely change the way it has been.