Why workflows?
Workflows are used to model business processes. Anything you can think of from order processing, car rentals approvals, and the like are process-oriented systems that can benefit from workflows. You might wonder why you need workflows because you can clearly implement these applications using classical programming techniques. While that is true, Workflows allow you to model your solution visually, and allow you to use a powerful mix of imperative code and declarative activities and rules to get the same task done. At run-time, the graph of activities that you created is interpreted and execution proceeds along the links you defined. The ability to do declarative programming for complex processes allows you to model processes that are very dynamic in nature in a much more simpler and maintainable way.
For a detail introduction, read http://msdn.microsoft.com/winfx/reference/workflow/default.aspx?pull=/library/en-us/dnlong/html/wfintro.asp
