Personal Messages, or PM for short, are the forum equivalent of internal emails. These are restricted to the recipients specified (existing users on the forum) as would be done with emails, and can also allow for a BCC (not available by default, but can be selected on the PM page), and can be replied to or forwarded as the recipient likes.
It is also possible to organize the PMs into folders, and delete ones that are no longer required. There is a limit of how many can be stored, and once a user reaches that limit (which also includes saved "Sent" PMs), they must delete some of their messages before they can receive any more PMs.
Abuse of this system is not tolerated at TXU, and abuse or Spam sent within the PM system is met with either infractions or banning.