Sarfaraz Hussain explains how some of the pieces of the Hadoop ecosystem fit together:
Once it verifies that everything is in place, it will assign a Job ID to the Job and then allocate the Job ID into a Job Queue.
So, in Job Queue there can be multiple jobs waiting to get processed.
As soon as a job is assigned to the Job Queue, it’s corresponding information about the Job like Input/Output Path, the location of the Jar, etc. are written into the temp location of HDFS.
Read the whole thing.