The Joker first appeared in the very first issue of Batman's own comic book, in spring of 1940. From the very beginning, the grinning, green-haired, chalk-skinned villain was popular with comic readers. As time went on, the character became more maniacal and murderous, until by the time of Frank Miller's Dark Knight Returns miniseries in the mid-80's, he was an absolute sociopath.
The most recent (and probably the most influential, for some time to come) treatment of the Joker was in the film The Dark Knight, where actor Heath Ledger portrayed the Clown Prince of Crime. The treatment was deadly serious, however, in highly realistic, very violent scenes; and when Ledger died prematurely before the film's release, the mystique surrounding this film version of the Joker only increased.
For a brief time in mid-1970's, the Joker had his own comic book series.