A message board normally refers to a website where users post comments or questions.
Bulletin board systems (BBS) were the first examples of message boards, but they were not websites. BBS was based around the telnet protocol where users logged into a server and typed commands to view information or post their messages. BBS has existed on the internet almost since its inception and is probably the first example of social media.
The content in a message board is typically categorised and structured in a linear fashion.