The terms both come from physical-world analogies that early computer users found intuitive.
Bulletin Board Systems (BBS) came first, emerging in the late 1970s. The name was a direct metaphor for the cork bulletin boards you’d find in offices, schools, and community centers — physical boards where people could pin notices, announcements, and messages for others to read. The first BBS, CBBS (Computerized Bulletin Board System), was created by Ward Christensen and Randy Suess in Chicago in 1978. Like its physical counterpart, you’d “post” something for others to come read later. Access was through dial-up modems, and users would call in one at a time (or a few at a time on multi-line systems) to read and leave messages.
Forums drew on an even older metaphor — the Roman forum, the public square or marketplace where citizens gathered to debate, discuss civic matters, and exchange ideas. The word had already been used in English for centuries to mean any place of public discussion (legal forums, town hall forums, etc.). When online discussion spaces evolved beyond the simple BBS model — especially as the web emerged in the 1990s — the word “forum” felt like a natural fit for a place where many people could gather and hold structured, threaded conversations on various topics.
The two terms overlapped for a long time and were often used interchangeably. BBS culture was more associated with the dial-up era (pre-web), while “forum” became the dominant term for web-based discussion boards in the late 1990s and 2000s — platforms like phpBB, vBulletin, and eventually Reddit-style systems.
In short: bulletin board was a practical, literal metaphor for posting messages; forum was a grander, civic metaphor for public discourse. The latter won out in common usage as the internet matured.