Is there a global documentation about all the classes ?
No, not yet. It is planned but we simply did not had the time to write it.
Basically forms are inherited from tform_actions class and lists from listform_actions class. You can override subfunctions of the form calss similar to the list class, so you can read even data from other sources then mysql. See e.g. the admin/server_config_edit.php file which reads the data that is displayed in the form from a ini style text blob field in mysql (it could have been a file as well or also ldap if you use ldap functions to get data in form of a array / hash in php.