Are you sure you're not looking for AbstractWizardFormController vs. SimpleFormController ?
AbstractFormController is the support class provided by the framework for form controllers in general.
Simple is the implementation for the trivial case: a form submitted in one step -- it has a form view and a success view. The controller selects the view based on the request method: GET or POST (well, it's actually a lot more than this but you'll find that in the Javadoc).
AbstractWizardFormController on the other hand allows creating a web wizard by populating the command object in several steps (the classic Next, Next... Finish).
But
you should be aware that in Spring 3.0 all the *FormControllers have been deprecated in favor of annotated controllers.