Joshua Thjissen has a post on his site that wants to help you understand the basics of Symfony2 forms including how to build them, extend them and the modules they're made up of.

To actually use Symfony2 forms, all you need to do is read some documentation, a few blog posts and you’ll be up and running in a couple of minutes. Understanding Symfony2 forms however, is a whole different ballgame. In order to understand a seemingly simple process of “adding fields to a form”, we must understand a lot of the basic foundation of the Symfony2 Form component. In these blog posts, I’ll try and give some more insights on this foundation.

He starts by explaining the three main steps in the typical form lifecycle: building the form itself, populating and validating data and rendering the form to the waiting user. He then gets into some of the basics of using forms and the types of objects that make them up. He includes examples of creating a simple form, the YAML configuration it compiles to and the functions used to build, render and set options on the form. He finishes up the post looking at form inheritance, extending the form types and where the "ResolvedFormType" comes in to play.