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.

Ward Christensen coined the term "Bulletin Board System" as 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. By "computerizing" this method of communications, the name of the first BBS system was born: CBBS - Computerized Bulletin Board System. 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, social network services and other aspects of the Internet.

A bulletin board (pinboard, pin board, noticeboard, or notice board in British English) is a surface intended for the posting of public messages, for example, to advertise items wanted or for sale, announce events, or provide information. Bulletin boards are often made of a material such as cork to facilitate addition and removal of messages, or they can be placed on computer networks so people can leave and erase messages for other people to read and see.

Bulletin boards are particularly prevalent at universities. They are used by many sports groups and extracurricular groups and anything from local shops to official notices. Dormitory corridors, well-trafficked hallways, lobbies, and freestanding kiosks often have cork boards attached to facilitate the posting of notices. At some universities, lampposts, bollards, trees, and walls often become impromptu posting sites in areas where official boards are sparse in number.

Internet forums are becoming a global replacement for traditional bulletin boards. Online bulletin boards are sometimes referred to as message boards. The terms bulletin board, message board and even Internet forum are interchangeable, although often one bulletin board or message board can contain a number of Internet forums or discussion groups. An online board can serve the same purpose as a physical bulletin board.