A joke is a funny story or line that's meant to make people laugh. When you're good at telling jokes, you're the life of the party (and you're probably not telling knock-knock jokes).
Many jokes have a "punch line," the part that either makes your listeners guffaw or groan, depending on how funny it is. The rest of the joke leads up to this climax. Another kind of joke is the "practical" sort, a prank, gag, or trick you play on someone. In the 1660s the word was spelled joque and was considered slang. It comes from the Latin root iocus, "joke, sport, or pastime."