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.