It is believed that the term "Joker" comes from a mispronunciation of Juker, the German/Alsatian name for the game Euchre. The card was originally introduced in about 1860 for games of that family to be used as the highest trump. Catherine Perry Hargrave documents jokers from 1862 and 1865 in her book A History of Playing Cards. The 1862 card has a tiger on it and the label "Highest Trump", while the one from 1865 is inscribed "This card takes either Bower" and "Imperial Bower", or "Highest Trump Card". So it may be that Euchre was the game for which the Joker was invented, not Poker because part of this confusion on the issue may be because both games spread simultaneously northward on the Mississippi.
The Joker came to be represented as a clown or court jester by the 1880s and often still is today. There are usually 2 Jokers per deck, often noticeably different. For instance Bicycle Playing Cards prints their company's guarantee claim on only one. More common traits are the appearance of colored and black/noncolored Jokers. At times the Jokers will each be colored to match the colors used for suits; there will be a red Joker, and a black Joker. In games where the jokers may need to be compared, the red or full-color joker usually outranks the black-and-white one; if the joker colors are similar the joker without a guarantee will outrank the guaranteed one.
It's a euphemism popularized by the Insane Clown Posse on their first EP, Beverly Kills 50187.