At its core it is an event-procesing engine that is based on pubsub converntional patterns.
Wikipedia define pubsub as:
“Publish/subscribe (or pub/sub) is a messaging paradigm where senders (publishers) of messages are not programmed to send their messages to specific receivers (subscribers). Rather, published messages are characterized into classes, without knowledge of what (if any) subscribers there may be. Subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what (if any) publishers there are. This decoupling of publishers and subscribers can allow for greater scalability and a more dynamic network topology.”
Common flow in BizTalk is once a meesage is recieved by an adapter, is is run through pre-processing in a pipeline, before being data transformed in maps, then being published to the messagebox.
* IMPORTANT NOTE: Once a message has entered the messagebox it is immutable (which means CANNOT be changed).
Because of the the nature of pub/sub patterns a bit a latency is introduced, which BizTalk makes up for in the starabilities (durability, reliability, and scalability).
The most dynamic use for BizTalk is in SOA or Service bus architecture. In this archtecture itinerary services was introduced, which basically applied a message with a purpose concept.
What is this message you talk about?
It is the data processed through the BizTalk messaging engine (IE. XML document, Word Document, Flat file, etc.). The message basically contains 2 critical parts message type and context. The message type defines it within the bus and is typically a XML namespace and root node. The message context is a set of name/value pairs.
How does a message get to and from BizTalk?
BizTalk uses recieve locations which are defined as endpoints, which use a particular adapter which knowns how to receive that specific message. These receive locations have a particular receive pipeline associated with them, which can be used to modify the message before it becomes immutable. Now, a receive port is defined as containing more then one receive location, which apply XSLT maps to the message prior to being published to the messagebox.
BizTalk used send ports and orchestrations (2 real type of subscribers within BizTalk), send ports provide a means of transporting messages from the bus and an orchestration provides an executable business process, which uses the message to complete workflow operations.