Several of the applications that we've built or are currently working on in the OSS group involve the processing of workflow in some form. If you peruse the Web you'll find there are a variety of types such as "activity-based" or "content-driven". Microsoft Windows Workflow Foundation supports two types: "Sequential" and "State Machine" (http://msdn.microsoft.com/en-us/library/ms468447.aspx). There is enough material out there though to make your head spin. Being that I'm not an authority on the subject I thought I'd explain in a simple-to-understand format the two types that we've used.
Hierarchical: workflow events are fired one at a time and they operate in a very sequential fashion...one waits on the other. Example: I fill out a form and submit it to my boss for approval. Until he approves or denies the request, the entire transaction is in a holding state. Once he approves it, it then gets submitted to his boss. If he denies the request, it comes back to me. After all approvals have been gathered, the transaction is considered complete.
Linear: here workflow events all fire off on different threads (asynchronously) and are independent of one another. They must all be completed though before the transaction can be considered complete. Example: I put in a request to get a widget ordered. This order requires the participation of four different groups to perform an action. So, requests are sent to all four groups and they all complete their task independent of one another. Once all four are completed, the widget can be processed.
Hopefully those two approaches make sense. If you have more to add to this topic, I welcome your feedback.