Any difference in meaning between those two terms, or are they exactly synonymous?
At the SDMB they are different. The message board is the set of all fora. General Questions is one forum at the Straight Dope Message Board. Another forum would be “About This Message Board”.
Most boards running on BBBoards, uBB, or VBB allow for a main board menu with lists of individual fora divided into such categories as the administrators see as appropriate. Threads can be started by users in each of the individual fora.
There may well still be BBSes that retain a single listing of subjects to be read and commented on, but I haven’t encountered one in many years.