Definition of Dressage: 
"Dressage (french for “training”) refers to training a horse to be thoroughly supple, balanced, and responsive." 
-Encarta '95 

This discipline consists of the rider using both natural and artificial aids to guide the horse through different movements ranging from simple school figures (eg. circles and serpentines) to difficult lateral movements (eg. half pass, canter pirouettes etc.). Throughout the world when competing in this discipline, there are different levels in which you can compete and progress based on both rider's and horse's experience. The lowest level on the competition scene is Preliminary: where the horse must know how to do simple figures at walk, trot and canter. The highest level in the competition scene is Grand Prix Special: where the horse must do very difficult movements with what may appear little or no effort by the rider. For a proffesional to train a horse to Grand Prix level it could take over 8 years. 

When competing, the horse and rider is judged by one or a panel of judges (depending on what level you are competing at) on three simple criteria: Suppleness, balance and responsiveness. Without these 3 fulfilled it is impossible or highly unlikely to achieve a score of 8-10 on your movement. 

The essence to dressage is harmony between horse and rider. The horse and rider must form into 'one'. Without this 'harmony' dressage is simply another equestrian discipline, but with it, this discipline is, in itself, an art form.