Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is the Unix derivative distributed by the University of California, Berkeley starting in the 1970s. The name is also used collectively for the modern descendants of these distributions.
Mac OS X is a derivative of the original BSD Unix distribution. This means Mac OS X is essentially a flavor unix, and you can use the Terminal utility provided with the OS execute unix-style commands. Also, the file systems owner-group-everyone permission scheme is based on unix.
Finally, where as Mac OS 9 used colons (:) to separate directories in path name, Mac OS X, being an derivative of BSD, uses slashes (/)