By definition, trail running means running on trails, but the concept has more aspects.
Physically it’s a very varied form of exercise. You use the whole body when you contend with roots, rocks and branches, climbing up and down steep gradients or force your way through the bushes. The ground is seldom smooth so you develop your sense of balance. You avoid the monotonous stride from road running – a changing and varied stride is more gentle on the knees and joints.
Mentally, it’s a great feeling of freedom. The running will be spontaneous, you are constantly discovering new places and views of the terrain. When crossing terrain at high speed you can discover the “hunt or be hunted” feeling which is the origin of all running.
To become a good trail runner, you need stamina, strong legs and, sometimes, basic knowledge of orienteering.