A King Cake is much like a pastry, particularly a cinnamon bun. One main difference is the King Cake does not have raisins. The cake itself is often round, although square or oval with a hole in the center is fine. The dough is often covered with cinnamon which is then twisted into the cake as layers. The amount of cinnamon one finds in a King Cake varies by the bakery from which it came.
The name has to do with the Feast of the Epiphany, which celebrates the arrival of the Magi, often called the three kings, to visit Jesus. They had to find Jesus, so a small doll representing Jesus as a baby is hidden into the cake. Some bakers add the doll, others allow the customer to do so. If the baker hides the doll it is well concealed.
The cake is decorated with granulated sugar in the three Mardi Gras colors of purple, green, and gold. These symbolize justice, faith, and power, respectively. The sugar is not mixed, but the colors are applied in swaths of about four-inch widths. So, there is a purple section, then a green section, then a gold section, then repeat as many times as needed. Another decoration, which is optional, is white icing that often is drizzled on without fully covering the cake. But there is no real requirement. Some bakers add so much icing it practically closes the center hold and merges with icing from the other side of the cake.