A joke is a series of words, phrases, or occasionally actions (as in physical humor) intended to produce laughter, and not offense. Generally speaking, a joke will contain some exaggeration or absurdity which is difficult to take seriously. Many jokes rely on wordplay or "puns", others on current events and still others on the preconceptions of the audience in order to invoke the desire effect.
Jokes can be differentiated from mockery in that jokes are innocent attempts at humor, while mockery includes intent to harm, ridicule, or embarrass. As such, jokes are typically good-natured, while mockery is not.
A joke typically consists of two parts. The first part is referred to as the "setup." The last sentence of a joke is called the "punchline." This is the part you are supposed to laugh at, although it is rarely as funny as the joke-teller thinks it is. A one-sentence joke is called a "one liner."