The oldest "Who's Who" is the British biographical directory that has been published annually since 1849 by A & C Black. Another notable "Who's Who" is published by the American Marquis company. It has a long history as well, starting from 1899.
"Who's Who" is the title of several biographical directories by various authors and published by various companies. It's a general title for a publication that contains biographical information on a particular group of people. For example the Marquis "Who's Who" includes a number of books with varying topics, such as "Who's Who in America" and "Who's Who in the World."