BSD is an acronym for Berkeley Software Distribution, a flavor of Unix operating system that reigned in the 1970s and 1980s. It had the typical Unix features like "timesharing" (for multiple users and processes) plus BSD enhancements like the fast file system, virtual memory, that text editor called vi and the first TCP/IP network implementation. BSD became popular at universities because anyone with a (then-cheap) Unix research license from AT&T could run it. In 1982, students obtained the latest version, 4.2BSD; they noticed the inclusion of a new game called Rogue. Thus BSD enabled the spread of the entire roguelike genre of games.
BSD became free in the early 1990s with the removal of proprietary Unix code, but then in the University of California at Berkeley stopped developing it. Many installed instead GNU/Linux.