If you've stumbled across this site I think you probably already know. In that case click here to return to the main page. Otherwise I will explain briefly. Also known as a discussion forum, a message board is a place where users can post a thread and get replies to it. Basically ask a question and other people will reply to it. The formats of boards differ. Some you have to register with, while others you simply add a posting name. Also, in some the threads are arranged in chronological order while in others they are arranged by activity level.
Enough info.