After reading some of your other posts, you seem like a newbie to Java, JMS and Google, but ho hum...
JMS is a mechanism for getting two or more java applications to pass messages to each other. The messages can be strings, bytes or even objects provided they are serializable.
Among JMS's strenghts are that it can be transactional and persistent, meaning that your can guarantee once and once only delivery of a message.
Disadvantages are that JMS by itself does not support languages other than Java, although some JMS vendors have written client libraries for this.
I suggest you take a look around java.sun.com for more information and code samples.