an operating system may provide facilities for constructing user interfaces such as a graphics display driver or a keyboard driver however what we now call a user interface such as the macintosh or windows user interface is a congeries of libraries and application programs which use those os facilities a web browser like internet explorer or file manager such as the macintosh finder is only part of an operating system when that word is used as a marketer or product manager would use it rather than as a computer scientist would use it these are distinct uses of the word just as set means something different to a collector as in collect the whole set from what it means to a mathematician