Application Master performs the role of negotiating resources from the Resource Manager and working with the Node Manager(s) to execute and monitor the tasks.
Application Master requests containers for all map tasks and reduce tasks.Once Containers are assigned to tasks, Application Master starts containers by notifying its Node Manager. Application Master collects progress information from all tasks and aggregate values are propagated to Client Node or user.
Application master is specific to a single application which is a single job in classic mapreduce or a cycle of jobs. Once the job execution is completed, application master will no longer exist.