IBM WebSphere MQ Cluster is the deployment of the IBM WebSphere MQ with clustered queue configuration instead of the distributed one (see IBM documentation for the difference between these configurations).
IBM MQ Series, now branded as Websphere MQ, is IBM's Message Oriented Middleware (MOM) offering. It allows independent and potentially non-concurrent applications on a distributed system to communicate with each other. MQ is available on a large number of platforms including z/OS, UNIX (AIX, HP-UX, Solaris). Linux, and Microsoft Windows.
Websphere MQ provides assured one-time delivery of messages across a wide variety of platforms. The product emphasizes reliability and robustness of message traffic, and ensures that a message will never be lost.