Oh come on, don't tell me you actually believe that crap. Those old forum fairytales!Bordeaux
BBS is short for a Bulletin Board System. In the American version of .hack, it is often just called "The Board". It is an internet forum where users can exchange information or discuss topics of interest. Users of The World use its official BBS to give and receive gameplay help, discuss current events, or arrange a time and place to meet. Other forums unaffiliated with CC Corp exist as well, and usually have more lenient rules about content.
In the original series, the BBS consisted of a single board. The Official "The World" forums. Information was shared by players regarding all aspects of the game. The forums were heavily moderated by CC Corp employees such as Lios and because of this posts involving cheats or other things against The World's terms of service were quickly deleted. During the Twilight Incident the forums were watched carefully, and any posts hinting to trouble inside of the game were erased almost immediately.
While the official forums still exist in The World R:2 there are many other forums that exist as well. The most popular is Community BBS, hosted by Yomoyama, a widely-used ISP. The subforums on Yomoyama range from news discussion forums, to rumors forums, and even an artwork forum named "Apkallu". These other forums are not moderated by CC Corp, and so even topics about illegal aspects of the game are posted frequently.