What is the name of the outdoor sport activity that was started in French ghettos and involves people running up walls and jumping off railings and buildings?

Answer 1

It's called Parkour.

from Wikipedia

(or le parkour) is the physical practice of traversing elements in both urban and rural settings, although it is sometimes seen as a philosopy. The goal is to move from one point to another as quickly and efficiently as possible. This discipline was created in France, in Sarcelles, Lisses and Evry by David Belle, Sébastien Foucan, and the founding members of the Yamakasi. It is inspired by "the natural method of physical education" by Georges Hébert who first saw this form of movement done by Africans in Congo. It was then spread worldwide by films, television reports, and amateur videos on the internet.

The term freerunning is sometimes used interchangeably with parkour. While parkour aims to enable the practitioner to be able to move quickly and efficiently past obstacles, freerunning has a greater emphasis on self-expression within the environment. Freerunning includes tricking moves such as aerial rotations and spins, while the purist definition of parkour founder David Belle would not consider these part of parkour because the moves are merely showy, not efficient, and do not help the participant to get from place to place. Although Sébastien Foucan co-founded parkour, his philosophy differed and so he is generally associated with freerunning (see below).

A practitioner of parkour is called a traceur if male, or traceuse if female. The word is most likely derived from parisian slang verb "Tracer" which means "to move fast" or "to hurry". In proper French "traceur" is an adjective qualifying something that leaves a trace or a trail behind it.