What are they and why do we wear them? Well, Let's find out...
Petticoat refers to any separate skirt worn under a dress. Crinolines and hoops, or hoop-skirts, are all types of petticoats. There is a very interesting history on petticoats, crinoline, and hoops and how they all evolved in women's fashion. I will not go into that now (even though it does fascinate me), if you find it interesting as well you can read more about that here.
But, alas, I must focus on the present day uses for petticoats, and how they apply to the bridal world.
The term crinoline can refer to the actual stiffened material, or a separate underskirt that helps support and retain the shape of a gowns skirt. Most wedding gowns have built in crinoline that give the skirt shape, but if that still does not create the desired fullness, you can always have more crinoline sewn into the gown, or purchase a separate crinoline to wear with the gown to achieve the shape that you want.
Hoops originated as a modest-sized mechanism for holding long skirts away from one's legs for two main reason; to keep ladies cooler in hot climates, and to keep from tripping on ones skirt during various activities. Old time hoops were made of metal, making them extremely heavy and cumbersome. Thanks to invention of plastics, today's hoops are much lighter and flexible, and not nearly so heavy. Hoops can be worn under a-line and ballgown dresses. Depending on the size of the hoop they can add more fullness to the skirt, or they can just hold the shape without adding much volume. Today's brides (and prom girls), just like the women that used hoops when they were first invented, are adding a small hoop under their dresses for the comfort of not having fabric clinging to their legs, and tangling around their feet. And don't worry ladies, the plastic hoop will lay flat as you sit, unlike metal hoops that had a tendency to "pop up" if a lady did not sit just right.