It's explained here. Note that SimpleFormController is a concrete implementation of the AbstractFormController.
Concrete FormController implementation that provides configurable form and success views, and an onSubmit chain for convenient overriding. Automatically resubmits to the form view in case of validation errors, and renders the success view in case of a valid submission.
The workflow of this Controller does not differ much from the one described in the AbstractFormController. The difference is that you do not need to implement showForm, processFormSubmission, and renderFormSubmission: A form view and a success view can be configured declaratively.
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.