A tutu is a skirt, originally worn by ballet dancers but now also widely seen in fetish clothing. It is fairly short and sticks out, either because it is supported by wires or because it is made of very stiff, starched material. In consequence, it is from certain angles very revealing, often more so than the shortest of ordinary miniskirts. This was originally to allow female dancers maximum freedom of movement, but, especially in a fetish context, it has voyeuristic aspects.
A tutu is usually made from see-through material, hence those made from just one or two thicknesses of material are fairly transparent unless they are lined. However, they are often made of five, six or even seven layers, and these tutus are opaque.
A ballet tutu is a skirt worn as a costume in a ballet performance, often with attached bodice. It might be single layer, hanging down, or multiple layers starched and strutting out. It tends to add a floating motion when ballerinas dance in them. Tutus have crossed over into mainstream fashion and are common in the goth sub culture. They are a frequently seen in clubs such as Torture Garden.