Workflow Scalability and Performance in Windows SharePoint Services 3.0 article, authored by David Mann and Rohit Puri has been published recently in MSDN.
Though it focuses on workflow scalability and performance, it also gives an overview of workflow in Windows SharePoint Services and the types of situations in which you might use it. Do read the "Workflows vs. Event Receivers" section to understand the similarities and differences between workflows and event receivers and the "Workflow Processing Overview" section to understand how they are processed. It also covers the important Workflow Configuration settings - Throttle, Batch size, Timeout, Workflow timer interval and Workflow AutoCleanUpDays.
In the "Recommendations" section, some tasks to achieve maximum performance are discussed, which need to be done regardless of whether you are running custom or built-in workflows.
An informative article and recommended for anyone developing SharePoint workflows or planning to develop them.