Think about what happens when you buy something on Amazon; register on ebay; book a hotel room online; transfer money from a bank account. If wireframes are the "public facade" of a customer experience like this, then a process model is the mess of wires hidden behind that facade, linking everything up in a coherent flow of information.

Diagrammatically, process flows resemble flow charts and diagrams, entity relation schemes and the various species of interrelationship digraphs.

Process modeling is contingency handling, by another name. It's the job of thinking iteratively and deeply about a whole series of "What happens if...?" questions, and then generating answers to those questions. Process modelling is used extensively to model web applications, for ecommerce offerings, and for any complex online proposition where there are multiple - and multiply contingent - actions possible from a given web page.

Though there are similarities with finite state grammers, process flows are not as complex or as mathematically formal, although like a finite state grammer, a process flow always has a sense of movement or progression; states and descriptions; and decisions and transitions between states. The important thing is capturing the contingencies, and representing the flow.

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