1. Who this one is and who that one is; who the different ones in a group of people are or what their names or positions are.
It is hard to tell who is who in the parade because everyone in the band looks alike.
It took the new teacher a few days to remember who was who in the class.
Compare: WHICH IS WHICH.
2. Who the important people are.
John didn't recognize the champion on television. He doesn't know who is who in boxing.
After about a year, Mr. Thompson had lived in this town long enough to know who was who.
Compare: WHAT'S WHAT.