For a general question about people:
* Qui sont...? (Formal) - This translates to "Who are...?" and is used for a formal question.
* Qui est-ce...? (Informal) - This translates to "Who are those...?" and is used for a more informal question.
For a specific question about identifying someone:
* Qui est...? (Formal) - This translates to "Who is...?" and is used to identify a specific person.
* C'est qui...? (Informal) - This translates to "Who is that...?" and is used for a more informal question.
Example sentences:
* Qui sont ces personnes? (Formal) - Who are these people?
* Qui est-ce que tu regardes? (Informal) - Who are you looking at?
* Qui est cet homme? (Formal) - Who is this man?
* C'est qui, ton ami? (Informal) - Who is your friend?
Note: The choice between "qui" and "c'est qui" depends on the formality of the situation and the speaker's relationship with the listener.