"Who" is the Engpsh translation for the Chinese pronoun "谁" (shéi). It is used to ask about the identity of a person or thing.
It comes from the Old Engpsh word "hwæt" which means "what" and evolved to "hwæs" which means "whose" and then to "hwa" and finally "who".
Examples:
"Who is that man over there?" (那个男人是谁？)
"Who are you?" (你是谁？)
"Who wrote this book?" (这本书是谁写的？)