Job Management Systems has several names Scheduler, Broker, Middleware,
Batch and Queuing System, Core middleware, User level middleware and so on.
There will be many names to come for the future, we will leave it at this stage.
Grid Middleware and JMS are one and the same to some extent (I would say!).
JMSs' work is to the receive the job/work from the user, schedule the job on
appropriate resources, monitor the progress and return the results back to the user.
JMS gives an end user/application developer single system image of the underlying
resources, persay give an experience that user is sitting in front of a one powerful
computer.
With respect to the Cluster, the hardware and software setup gives Single System Image(SSI).
This SSI characteristic is given by Job Management System for the Grid Environment.