A shepherd is a person who tends to, feeds, or guards sheep - especially in flocks. The word may also refer to one who provides religious guidance.
Last year I used this word to refer to those who took care of me in the hospital two years ago. They tended to me, they fed me, and they guarded my very existence. A good friend questioned my usage because the term implied "leading." To me there was an obvious reference in that interpretation. "He leadeth me beside still waters" or "He leadeth me in the paths of righteousness for his name's sake."
That was, however, too literal for me. I interpreted the psalm as describing God's protection and care, perhaps guidance and care, and that is exactly what my Shepherds did for me.