According to Wikipedia: Running is a means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. It is simply defined in athletics terms as a gait in which at regular points during the running cycle both feet are off the ground. This is in contrast to walking, where one foot is always in contact with the ground, the legs are kept mostly straight and the center of gravity vaults over the stance leg or legs in an inverted pendulum fashion. A characteristic feature of a running body from the viewpoint of spring-mass mechanics is that changes in kinetic and potential energy within a stride occur simultaneously, with energy storage accomplished by springy tendons and passive muscle elasticity. The term running can refer to any of a variety of speeds ranging from jogging to sprinting.
According to Wikipedia: Trail running is a sport which consists of running and hiking over trails. It differs from road running and track running in that generally takes place on hiking trails, often in mountainous terrain, where there can be much larger ascends and descends. It is difficult to definitively distinguish trail running from cross country running. In general, however, cross country is a IAAF governed discipline that is typically raced over shorter distances (rarely over 12 kilometers), whereas trail running is loosely governed, and run over longer routes.
Road or Trail?