Java Message Service API (the JMS API)

General idea of messaging

Messaging is a form of loosely coupled distributed communication, where in this context the term 'communication' can be understood as an exchange of messages between software components. Message-oriented technologies attempt to relax tightly coupled communication (such as TCP network sockets, CORBA or RMI) by the introduction of an intermediary component, which in this case would be a queue. The latter approach allows software components to communicate 'indirectly' with each other. Benefits of this include message senders not needing to have precise knowledge of their receivers, since communication is performed using this queue. This is the first of two types: point to point and publish and subscribe.

Java Message Service API Overview

The Java Message Service (JMS) defines the standard for reliable Enterprise Messaging. Enterprise messaging, often also referred to as Messaging Oriented Middleware (MOM), is universally recognized as an essential tool for building enterprise applications. By combining Java technology with enterprise messaging, the JMS API provides a powerful tool for solving enterprise computing problems.

Enterprise messaging provides a reliable, flexible service for the asynchronous exchange of critical business data and events throughout an enterprise. The JMS API adds to this a common API and provider framework that enables the development of portable, message based applications in the Java programming language.

The JMS API improves programmer productivity by defining a common set of messaging concepts and programming strategies that will be supported by all JMS technology-compliant messaging systems.

The JMS API is an integral part of the Java 2, Enterprise Edition (J2EE) platform, and application developers can use messaging with components using J2EE APIs ("J2EE components").

Version 1.1 of the JMS API in the J2EE 1.4 platform has the following features:

• Message-driven beans enable the asynchronous consumption of JMS messages.
• Message sends and receives can participate in Java Transaction API (JTA) transactions.
• J2EE Connector Architecture interfaces that allow JMS implementations from different vendors to be externally plugged into a J2EE 1.4 application server.
  

The addition of the JMS API enhances the J2EE platform by simplifying enterprise development, allowing loosely coupled, reliable, asynchronous interactions among J2EE components and legacy systems capable of messaging. As a developer, you can easily add new behavior to a J2EE application with existing business events by adding a new message-driven bean to operate on specific business events.

The J2EE platform's Enterprise JavaBeans (EJB) container architecture, moreover, enhances the JMS API in two ways:

• By allowing for the concurrent consumption of messages
• By providing support for distributed transactions, so that database updates, message processing, and connections to EIS systems using the J2EE Connector Architecture can all participate in the same transaction context.

See : 
http://www.oracle.com/technetwork/java/overview-137943.html

See also: Message-oriented middleware and Message passing

And a complete useful tutorial you cant miss :
http://download-llnw.oracle.com/javaee/1.3/jms/tutorial/1_3_1-fcs/doc/overview.html