WebSphere Message Broker, formerly known as MQSeries Integrator, part of the Application Connectivity (MOM) subcategory of products, provides messaging mediation capabilities--routing and transformation. It typically runs on top of WebSphere MQ.
There is now a WebSphere Message Broker 6.0 Information Center. The manuals are also available.
A good way to learn about how to integrate applications using a messaging system like WMQ is the book Enterprise Integration Patterns.
