The first time I went wedding dress shopping with my friend Kris is when I learned about the importance of the bustle. For the uninitiated, very simply, a bustle is when you take some extra material of the dress and gather it in a variety of ways. It not only helps keep extra fabric out of the way (ie for walking or dancing) but it can aesthetically transform the dress as well.
I thought this illustrated the concept beautifully:
(wedding images from here and here)
Like the rest of the dress, there are endless ways of doing the bustle and it really depends on the dress and the bride's style. I think the one above is an example of a "multiple point french bustle" (also known as an under-bustle), where you secure the fabric underneath at, well, multiple points and you get the little poufs. (For contrast, compare with a single point french bustle)
I'll keep an eye out for pictures of other types of bustles because I think they're a super fun way to personalize the dress. In case you're wondering, I haven't decided what kind of bustle I want for my dress because I haven't even bought The Dress yet. Still. Urgh.