Online Bulletin Boards are analogous to the pinboards we are all used to from offices, schools and so on. The difference between a bboard and a mail list is fundamental:
Mail arrives in an individuals mailbox, and the individual's attention is drawn to it.
Bulletins arrive on a bboard, and users decide whether or when they want to read that bboard, if at all.
Less fundamental is the protocol. Bulletin boards are effectively a single mailbox. Thus the overhead of delivery in terms of computer storage is much lower for a bboard than a mail list.