We are still actively working on the spam issue.
BBS refers to Bulletin Board Systems, which were the equivalent of forums in the pre-web age.
Typically a user would dial-in to a BBS (ring a number via their modem) and be able to view and reply to posts made by others who had dialed in. Modern BBS' use the telnet protocol to connect users. They are famous in the Cyberpunk community, though lost their mainstream use in the early 2000's. BBS' were famous for housing some of the world's greatest people, such as Richard Stallman, Dennis Richie, and many others. A large number of internet groups started on BBS, such as The Cult of the Dead Cow, The Church of the Subgenius, and Heaven's Gate. Congressman Beto O'Rouke, a former member of The Cult of the Dead Cow, has many ramblings you can now access and laugh at which have been archived from BBS systems under the name Psychedelic Warlord.
8chan had an unofficial BBS shared with Lainchan, which was available at https://slashtech.ch, running the Mystic BBS software on a NetBSD system. However, due to lack of resources and loss of interest, the owner ended up ditching the whole project.
BBS: The Documentary will give you a good perspective.
If you wish to connect to a direct-line, there are several clients you can use for this. However, the simplest to use is the UNIX
minicom command. The minicom program has an extensive selection of features involving interfacing with your modem.
If you are not connecting to a BBS via a dial-in, then you have several clients you can use to connect to the BBS via telnet and ssh. You can use the default clients included in your UNIX system, but you may miss out on other features. The UNIX telnet and ssh client does not support common transfer protocols on BBS'. These are XModem, YModem, and ZModem but many others exist. Further, the BBS may have support for ANSI Music, an unofficial extension of ANSI that requires a proper decoder. Running this on a standard telnet client is prone to crash the session.
Because of this, it is recommended to download specialty clients. Perhaps the most popular is the Syncterm Client, made to be compatible with Syncronet BBS'. This client also comes with a long list of BBS' already inside it.
Multiple BBS directories exist in the modern age, the most important being The Diamond Mine Online, which also hosts its own BBS. You can go here for not only resources but daily updated lists.
The SDF Public Access Unix System began as The Super Dimensional Fortress Macros BBS in the 1980's and in some respects still resembles a BBS system. SDF uses ssh to connect, as well as a direct-dial and a telnet connection.