Millionaire playboy Bruce Wayne is secretly the Caped Crusader—The Batman (first appearance: Detective Comics No. 27, May 1939: "The Case of the Chemical Syndicate"; and others).
When young Bruce's parents are gunned down in front of a Gotham City movie theater by a cheap thug, Joe Chill, Bruce devotes his life to fighting evil as the Batman (Detective Comics No. 33, Nov 1939: "The Batman Wars Against the Dirigible of Doom"; and others).
His young ward, Dick Grayson, is secretly Robin, the Boy Wonder (Detective Comics No. 38, Apr 1940: "Robin the Boy Wonder!"; and others). After many years, Dick is succeeded as Robin by Jason Todd (Detective Comics No. 526, May 1983: "All My Enemies Against Me!"; and others).
Superman and Batman first discover each others secret identities in Superman No. 76, May/Jun 1952: "The Mightiest Team in the World", and work together on a regular basis from that time on.