Even though there is already a little information about dressage on the Riding Styles page, I don't think that it quite does this sport justice. So, here is more about dressage! Note: If you are using a dial-up connection, the page may load slowly due to animation and video.
Below is a video of dressage at the professional level. It is from Google Video.
Glossary of Dressage:
Dressage can seem very complicated. Let's start out with some basic terms:
    At dressage shows, you will ride in a dressage arena. To the right is a preview of how a 60x20 m (standard) dressage arena looks. A small dressage arena is 20x40 m and is used in lower levels of dressage.
     At the start of a dressage test, the rider enters at A. There is usually one or more judges at C. (In higher dressage levels, judges may be placed at multiple places around the arena.) X is the center of the arena. The letters in the center (or red letters) are not shown in a real dressage arena.
Note: This small glossary does not include all the terms of dressage. Only the basic and common terms.
Passage- a slow motion trot (right)
Piaffe- an approach to trot in place (right, finished piaffe)
Pirouettes- a 360-degree circle that is almost in place (right, at the canter)
Freestyle- dressage test preformed to music
Two-tempi- Lead changes on every other stride (right)
One-tempo- Lead changes on every stride
Animations from Animated Guide to Dressage Netscape 2.0 or Microsoft Internet Explorer 3.0 required to view animations.
Read An Introduction To Dressage Here!
Want to Know More About Dressage? Check out these websites:
Freedom  and Regularity of the Trot and Canter
Young Rider's Serious Schooling
Sample Dressage Tests
Dressage On About.com
    You can wear a normal english showing outfit for dressage. In higher levels, a formal top hat and spurs are worn. Sometimes, you are allowed to carry a whip  and sometimes not. Ask someone working at the show before you ride with one.
     Your horse must have a snaffle bit for dressage and should not be wearing a martangle. Your horse should be
braided and very clean. Hoofblack if often used. The tail is not usually braided.
     At dressage shows, a dressage saddle is not required. However, you must use and english-style saddle. Dressage saddles usually have a deeper seat. This helps riders keep their place in the saddle.