From JoyofBaking.com:
Bread Pudding is an old fashioned dessert that had humble beginnings in 13th century England. It was first known as a "poor man's pudding" as it was made from stale leftover bread that had been simply moistened in water, to which a little sugar, spices and other ingredients were added. Today, we still make our bread puddings with bread but the breads we use are often made especially for this pudding and the types are wide ranging; breads like brioche, challah, croissant, panettone, French, Italian and sometimes even raisin bread or scones. We are also not content to simply soak the bread in water, instead we use a rich mixture (really a custard) of milk (or cream), eggs, sugar, vanilla, and spices. Sometimes even nuts, chocolate, zests, alcohol, candied, dried or fresh fruits are added for more flavor and texture. The end result is a rich, creamy, decadent dessert that has now made its way onto the dessert menus of many fine restaurants.