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Open MQ conforms to the JMS (Java Message Service) specification, which defines the basic concepts of how it works. I recomment you take a look at the JMS tutorial
In particular, take a look at the section on [Basic JMS API concepts|http://java.sun.com/products/jms/tutorial/1_3_1-fcs/doc/basics.html] .
The key thing to understand is that the sender of a message doesn't send it directly to the receiver of the message. Instead the sender sends the message to an abstract concept called a "destination". The receiver receives it from that destination. This way, the sender and receiver don't need to know about each other directly. There are two types of "destination": queues and topics, depending on whether you want each message to go to just one receiver or multiple receiver.
So the answer to question 1 is that a message sent to a particular destination will only be delivered to a receiver who subscribes to the destination. And if your question is about security rather than message delivery, then yes, open MQ can control access to destinations using user/password authentication,
As for question 2 (first part) the basic answer is yes: the client registers a callback method with Open MQ. When a message is received, that method will be called.
I didn't understand the second part of your question ("is it possible to ...push the message to clients other than clients receive the message?").