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