First appearance of Harley Quinn in the world of DC comics. The character has since had her own comics and graphic novels.
Originally, the script called for the Joker to appear in drag. The writers decided this was out of character for him and created a one-shot female character to fill this role instead. The character became extremely popular and was soon upgraded from one of Joker's goons to a major player in the Rogues Gallery: Harley Quinn.
Even though this was the first episode to be aired in which The Joker appeared in, it was actually the fourth episode with him to be produced.
Charles Collins' driver's license says he lives in Gotham Estates, New York. Assuming this is a suburb of Gotham City, this would put Gotham either in New York or in one of the five states adjoining it. Official DC canon puts it in New Jersey.