A joker is someone who loves jokes, and tells them all the time. Jokers are usually harmless, unless you meet up with Batman's archenemy, "The Joker." He's no joke.
A joke-teller or practical joke-player is one kind of joker, but you can also use this word when you're annoyed with someone who's acting ridiculous or foolish: "That joker in the convertible just ran a red light!" Card players might be more familiar with the jokers you find in a full deck, the extra cards that are either used as wild cards or removed when you play a game.