A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging messages with other users, either through electronic mail or in public message boards. Many BBSes also offer on-line games, in which users can compete with each other, and BBSes with multiple phone lines often provide chat rooms, allowing users to interact with each other.

Originally BBSes were accessed only over a phone line using a modem, but by the early 1990s some BBSes allowed access via a Telnet, packet switched network, or packet radio connection.

The term "Bulletin Board System" itself is a reference to the traditional cork-and-pin bulletin board often found in entrances of supermarkets, schools, libraries or other public areas where people can post messages, advertisements, or community news.

During their heyday from the late 1970s to the mid 1990s, most BBSes were run as a hobby free of charge by the system operator (or "SysOp"), while other BBSes charged their users a subscription fee for access, or were operated by a business as a means of supporting their customers. Bulletin Board Systems were in many ways a precursor to the modern form of the World Wide Web and other aspects of the Internet.

Early BBSes were often a local phenomenon, as one had to dial into a BBS with a phone line and would have to pay additional long distance charges for a BBS out of the local calling area. Thus, many users of a given BBS usually lived in the same area, and activities such as BBS Meets or Get Togethers, where everyone from the board would gather and meet face to face, were common.

As the use of the Internet became more widespread in the mid to late 1990s, traditional BBSes rapidly faded in popularity. Today, Internet forums occupy much of the same social and technological space as BBSes did, and the term BBS is often used to refer to any online forum or message board.

Although BBSing survives only as a niche hobby in most parts of the world, it is still an extremely popular form of communication for Taiwanese youth (see PTT Bulletin Board System). Most BBSes are now accessible over telnet and typically offer free email accounts, FTP services, IRC chat and all of the protocols commonly used on the Internet.