Umbraco Forms Documentation
Documents how to work with Umbraco Forms (HQ's official forms builder) from both an editor and a developer standpoint.
You can purchase Forms from Umbraco Apps.
Extending Umbraco with the Forms section and keeping it up to date
Documents how to interact with the interface and build forms in a couple of clicks.
The Developer documentation covers working with Umbraco Forms from a developer standpoint. It covers retrieving data from Forms, it shows how to extend the system by hooking into the provider model, and finally it describes the available events and workflows you can use to extend or integrate Umbraco Forms.