I . who's ˈwho
people's names, jobs, status, etc. :
You'll soon find out who's who in the office.
II . ˌwho's ˈwho
noun
a list or book of facts about famous people :
The list of delegates attending read like a who's who of the business world.
ORIGIN: From the reference book Who's Who , which gives information about many well-known people and what they have done.