Running on natural terrain - we're talking forests, hills, etc. - is nothing at all like running on flat, paved concrete. But despite this fact many runners who switch from concrete to a natural trail don't change their running technique to adapt to the new terrain. This is not a good idea because it can cause serious injury.
To learn how to run on natural terrain safely, check out this video.
Scott Jurek goes over some common mistakes made while trail running.