The word éclair comes from the French word for a lightning bolt, because that’s how fast people eat them. Originally they were called pain à la Duchesse.
They are made of logs of pâte à choux, filled with crème pâtissière or crème chiboust (chocolate, coffee, vanilla are the classics sold in almost every boulangerie in the land, but you may also find orange, walnut, pistachio, caramel, etc), all topped with fondant icing.
Image by Meg Zimbeck