L'art du deplacement (The art of displacement), better known as le parcours, and parkour in English, is the practice of maneuvering with optimal efficiency to reach a determined destination, or to escape pursuit. The term "le parcours" comes from the name of Georges Hebert's strenuous parcours du combattant, an obstacle course now used in Western infantry training on which one must vault, crawl and climb to finish.
While the history of parkour is the stuff of legend, it is certain that it has two founders who started to do it together as a recreational activity. David Belle, one of these founders, is reported to have been taught the parcours du combattant by his father, a French war veteran who practiced it for training in Vietnam. The other founder, Sebastien Foucan, was a friend of Belle's from a young age, and began running with him on his own accord. While these two would part ways philosophically later on in life, it was they to determine the course of parkour's evolution.
In his "Natural Method of Physical Culture", Georges Hebert emphasized the importance of purpose in movement; he believed that the parcours du combattant was purely a practical exercise, and that it prepared its practitioners for mobilization in coincidental conditions. David Belle's running is exemplary of this philosophy; he sets a destination, and moves however he must to quickly and swiftly arrive there. His running is often composed of vaulting over obstacles such as cars, diving under guard rails, and rolling out of leaps from building tops and other difficult heights. This is the essence of parkour. Sebastien Foucan, however, has another approach to parkour, which most traceurs and traceuses (men and women who practice parkour, respectively) refer to simply as "free running." Foucan's style is more dramatic, and often his running has no particular destination. He climbs and jumps for the sake of sport, and regularly tricks, which is something that David Belle has never felt to be genuine parkour in spirit. Foucan is now recognized as the founder of free running, and is often considered uninvolved in Belle's practice altogether.
Parkour has transformed into a lot more than "displacement." It is a lifestyle; a culture; a philosophy. Sebastien Foucan, originally a traceur, recommends a specific diet, training schedule and sleep schedule to his students. To those who are serious about it, parkour is more than a sport, more than running -- it's identity. Many traceurs and traceuses claim to approach life and think about the world differently after they start to parkour; they become more goal-oriented, and think more critically about problems that they encounter in their everyday lives. Those problems are, after all, much like the obstacles that they have to face while running: Only surmountable with a connatural cooridation, but easily surmounted with that coordination.
Moreover, parkour has become a cultural phenomenon. It has, in a way, been adopted by American and European subcultures alike as an "extreme sport," much like skateboarding or bungee jumping. While this is probably not appreciated by David Belle or Sebastien Foucan, that is primarily how the observing public tend to see parkour.
References:
http://en.wikipedia.org/wiki/Parkour
http://en.wikipedia.org/wiki/S%C3%A9bastien_Foucan
parkour.net