Interestingly, the word "folder" was popularized by Mac in the 1980s. Like many things, it was adopted (ripped off) by Windows. People in both groups apparently decided that "folder" made more sense to the common man.
On Mac, both words are in common use. The word "directory" will be found more often if you look at internals or work on the command line. Use "man ls" on a Mac and it tells you:
ls - list directory contentsSimilarly for "man mkdir" you will get:
mkdir - make directories
Adventures in Computing / tom@mmto.org