Starting with version 9.5, DB2 switched from a process-oriented architecture to a multi-threaded system architecture. One of the reasons was to better exploit the current and upcoming HW architectures.
Now that customers are migrating to DB2 9.5 and DB2 9.7, some of them coming from systems with unstable future or high maintenance costs, questions often heard are about things like "db2sysc" or "db2fmp". The DB2 Information Center has a short overview of the DB2 Process Model as a good starter. An article on developerWorks, "How multithreaded architecture works in DB2 9.5", has some more information.