what is jms?

JMS also known as java messaging service is an api intended to make the component loosely couple  by providing a mechanism to communicate among java clients. Note that JMS  is a peer to peer facility.

It allows distributed applications to be loosely coupled, reliable and asynchronous. This technology is widely used in java based message oriented middleware.

In essence JMS has two qualities.

1.  reliable : JMS makes sure the message is delivered and delivered only once.
2. asynchronous : A JMS provider can deliver messages to a client as they arrive; a client does not have to request messages in order to receive them

JMS can be a candidate over a tightly couple technology like RPC. These are the desirable qualities of JMS.

1.  Does not depend on the information of interfaces of other clients.
2. Can operate even when one or more other peers are down
3. A component can send a message to another and continue it's operation even without an immediate response

How ever above is just a little introduction of jms. You can see more information at

http://docs.oracle.com/javaee/6/tutorial/doc/bncdr.html