Webform includes documentation on all the public hooks it provides for other modules to utilize. The file webform_hooks.php or webform.api.php file included with Webform is divided into two sections. The first section defines the hooks available to module developers. The second section provides the definition for a sample webform component. This is useful when you want to create a component type that doesn't already exist.
Note: Most of the hooks had previously been listed here but have been removed due to technical inaccuracies. The best thing is to read the documentation directly from the Webform source. You can read the documentation from an API website such as the one below, or just read the source of the file directly.
Documentation Source: http://drupalcode.org/project/webform.git/blob/HEAD:/webform_hooks.php