Many web applications consist of a mix of free browsing, where the user is allowed to navigate a web site as they please, and controlled navigations where the user is guided through a series of steps towards completion of a business goal.
Consider the typical shopping cart application. While a user is shopping, she is freely browsing available products, adding her favorites to her cart while skipping over others. This is a good "free browsing" use case. However, when the user decides to checkout, a controlled workflow begins--the checkout process. Such a process represents a single user conversation that takes place over a series of steps, and navigation from step-to-step is controlled. The entire process represents an discrete application transaction that must complete exactly once or not at all.
Consider some other good examples of "controlled navigations": applying for a loan, paying your taxes on-line, booking a trip reservation, registering an account, or updating a warehouse inventory.
Traditional approaches to modeling and enforcing such controlled navigations or "flows" fall flat, and fail to express the Flow as a first class concept. Spring Web Flow (SWF) is a component of the Spring Framework's web stack focused on solving this problem in a productive and powerful manner.