BBSes once played the role that the InterNet does today, in an age when there was no such thing as the WorldWideWeb, and the InterNet was still a government research project. BBSes were text-based. Connecting to them was like logging into Linux in text mode. After you got past the login prompt, the BBS would send a menu over the phone line. You would respond by typing a letter or number from the menu. For example, if the menu said E)mail, you would type "e" to send Email. SysOps tried to spruce up the plain text interface by adding colour. For those who had colour monitors, BBSes looked like those lighted, coloured pegboards that they sold to kids during the same era.
The bulletin board originally started out as strips of paper posted up in the supermaket; this type of bulletin board is still in widespread use today. When computers became more widespread at home (late 1970s/early 1980s), the bulletin board system (BBS) took off. A BBS was a computer with a MoDem,hooked up to a phone line, which accepted incoming calls and enabled callers to exchange electronic messages. The very first BBS provided no other functionality. Later on, BBSes would support file uploads and downloads (via the XMODEM protocol at first, and later on the ZMODEM protocol), "door" games, real-time chat with the system operator (SysOp), real-time chat with other users if multiple phone lines were installed, and Netmail. Netmail was transferred daily over a global, MoDem-based network called FidoNet that mimicked UUCP and even improved upon it. Some BBSes provided a service called Echomail, which was very much like UseNet and was built on top of Netmail.
An entire industry rose around selling software for running BBSes. Some of these programs evolved into ISPs in a box and are still being sold today. Others, like the powerful Synchronet BBS, have gone in the OpenSource direction. BBSes at the time ran mostly on single-tasking OperatingSystems, so a whole computer had to be dedicated to a BBS.
BBSes and FidoNet still exist today, while UUCP has been phased out in favour of TCP/IP protocols such as SMTP. Some of today's BBSes run on modern computers under Linux. Some modern BBS software, in fact, offers a Telnet interface to its services – a number of BBSes aren't even hooked up to a MoDem any more, and can only be reached over the InterNet.