So what is it? A tart made from 2 discs of flaky pastry filled with a mixture of ground almonds, butter and sugar. Trust me, it's really good. If you're not a great lover of the richness of the Galette you can buy a Gateau des Rois which is a brioche mix with candied fruit, pretty and less fattening.
Inside both of them is hidden what used to be a feve, a bean, now it's usually a pottery or china figurine of some sort. The tradition is that the youngest person present hides under the table and as the cake is cut calls out the name of the person to whom that piece should be given. He who finds the feve gets to wear the crown and be king.
The feast itself dates back to Roman days and includes the same traditions. Just goes to show nothing much changes...