What is Who?
Who is pron 1. (asking a question) which person or persons Who was the person that phoned? Who phoned? Who are you talking to? Who was she going home with? Who spoke at the meeting? 2. the person or the people that The men who came yesterday morning work for the electricity company. Anyone who didn’t get tickets early won’t be able to get in. There’s the taxi driver who took us home last night. Do you remember the girl who used to work here as a waitress? (NOTE: With an object, who can be left out: there’s the man I saw at the pub. When who is used as an object, it is sometimes written whom : Whom are you talking about? The man whom I saw in the office but this is not common. After who used to ask a question, the (verb) is put before the subject: Who is that man over there? but I don’t know who that man is over there.)
source: Easier English, Student Dictionary Upper Intermediate Level