DC Comics did a Con Away From Con clip with Greg Capullo (artist of Batman), Scott Snyder (writer of Batman) and Brian Azzarello (writer of Wonder Woman) answering this question: “Who is The Joker?” Among all of these great men, the best answer is this:
“Who’s Joker? Joker is…uh…that’s a question that shouldn’t be answered. I think… as soon as you start defining him, he loses his power. He is what ever he needs to be at a particular time. That’s who Joker is.”
This was answered by Brian Azzarello who really understood who and what the Joker is. He’s everything you fear and the fact that you can’t describe him is what makes the Joker powerful.
See the clip below.
Source- DC Comics