Imagine having to speak about a profession that no longer exits or is no longer called what it used to be (e.g. "beadsman," "friar," "apothecary"). The audience knows that the talk is about a dead profession, but they don't know yet what it is exactly and whether it was performed by a man or woman. Now, you want to ask them a question to ease them into the subject. The question asks if any of the audience knows about the profession.
Do you ask:
a. Does anyone know what an "apothecary" is?
b. Does anyone know who an "apothecary" is?
"What" suggests that "apothecary" could be something not someone, which may throw the audience out of balance (remember that they have very little background; they can get easily confused)
"Who" sounds as if I were asking about his identity or something