Combined training or eventing is a test of horse and rider in the English riding style over three phases: dressage (art, skill, grace), cross-country (stamina and courage) and show jumping (accuracy, control and jumping ability). It is truly an "equestrian triathlon." Developed from the roles of military chargers, now combined training is an exciting sport for professionals and hobbyists.

Source: http://www.eventingusa.com/about/whatis.htm