A mail.message represents any message sent or received in OpenERP; both mail, internal messages or notes. The recipients of a message are contacts (res.partner) and the author is an email (text) and/or a contact. The mail.message may send email to recipients, according to their user's preferences.

A mail.mail inherits mail.message and is used only for sending emails. (like mass mailing) Sometimes, a mail.message will create a mail.mail that inherits from itself if the message has to be send by email to external persons.

You should usually not use mail.mail but always use mail.message. A message will handle all the logic of delivering emails to followers, according to their preferences, etc.

You can get the complete documentation of this module here: http://doc.openerp.com/trunk/developers/addons/mail/