Complex sentence: It is a commonly used term for the administrator of a bulletin board system (BBS) or special-interest area of an online service or, historically, the operators of any computer system, especially a mainframe computer.
Split into simpler sentences: It is a commonly used term for an administrator of a multi-user website, such as a bulletin board system (BBS) or special-interest area of an online service. Historically, the term has also applied to operators of any computer system, especially a mainframe computer.