There are 26 bones in the human foot, and each has a role to play when it comes to running. There are basically three ways a runner's foot hits the ground: rear footstrike, mid footstrike, and fore footstrike. How runners should land is an area of much debate and research, with very few definitive conclusions. The argument continues about which is best, but the most effective is probably the one that comes most naturally.
Rear footstrikers -- runners who land heel-first -- put less stress on their calf muscles, and because most running shoes are heavily padded in the heel, the impact is reduced. But this also creates a braking effect, essentially bringing the runner to a complete (if infinitesimal) stop with each step.
Runners who hit the ground with the ball of their feet and heel touching the ground simultaneously may experience fewer foot injuries because the impact is distributed over a larger surface area. But this method might be a little slower than the rearfoot strike.
Finally, those who land on the forefoot apply less pressure on their knees and ankles because the bounce of landing on your toes creates a natural cushion. It's also fast, and it helps direct energy forward, which is why sprinters often employ this method. But this is also hard on the calves and may cause premature muscle fatigue for distance runners.