Microsoft has released a good KB article on what message journaling is, how it works, and how to use it.
Message journaling terminology
Bifurcation In the bifurcation process, the categorizer generates a copy of an original message to the journal recipient or to the object that is specified in Exchange System Manager to receive copies of the archived message.
XEXCH50 The XEXCH50 extension is an Extension to SMTP (ESMTP) extension that is used for relay of certain properties, such as envelope properties, message properties, and recipient properties. The PR_CONTENT_IDENTIFIER identifier in an XEXCH50 binary large object (BLOB) is set to the ExJournalData string. The ExJournalData string enables journal messages to be recognized as they move from server to server on the delivery path to the journal recipient. If the XEXCH50 binary large object is not propagated between Exchange Server computers, duplicate messages may result.
Journal Recipient The journal recipient is the mailbox or the recipient that all journaled messages are destined for. The journal recipient is the distinguished name (DN) in the msExchMessageJournalRecipient attribute of the journaled stores.