What is WebSphere Message Broker?

Formal definition  (non vendor definition, as IBM has no single simple definition about that product :) ): WMB is a middleware software using which you able provide interaction heterogeneous applications regardless formats and protocols, supported by that applications.
In simple terms: if you want  (or maybe someone put you to task)  to provide transferring data between different application that supported different protocols – you  may solve this task using WebSphere Message Broker.
Remark: Often you may see close placed terms Message Broker, ESB, SOA. We will discuss this definitions later. But now you have to understand – SOA  and ESB are the patterns (or concepts) while Message Broker – is a tool. You may made some solutions using tool WMB and this solutions will be build accordingly service oriented architecture (SOA). But not all solutions are made using WMB will be SOA solution.
WMB supports a lot of transport protocols:
• WebSphere MQ
• JMS
• HTTP , Web Services (REST and SOAP)
• File
• ODBC, JDBC
• Enterprise Information Systems (including SAP and Siebel)
• TCP/IP
Also it supports a lot of data formats:
• XML
• CWF(Custom Wire Format, binary formats)
• TDS (Tagged delimited string)
• industry standards (including SWIFT, EDI, and HIPAA)

Same time, WMB supports different integration patterns and mechanisms such: routing, transformation, filtering, enrichment, monitoring, aggregation, corellation.

So, one of the main feature of  WMB, that it already supports plenty transport protocols ,data formats and integration patterns. Developers have not to worry about it's. In most cases developer will not implement some protocols or data parsers  itself or include some 3rd party libraries or facilities, because its already supported by WMB.