A private message is a message sent through the forum to another member. A notice of the message appears in the member's sign in box in the upper right hand corner of the page. If the member being pm'ed has selected the option, then an e-mail will be sent to them informing them that they have a pm waiting to be viewed. The private message can be viewed in e-mail but can only be replied to through the forum.
An e-mail sent through the forum goes to the e-mail address we have on file for the member and may be replied to directly through e-mail.
Generally a pm is the better option as many will not open e-mail from an unknown sender.