Our ancestors used to run to chase down food, like gazelles and antelope, which means we were born to run (kind of like the book "Born to Run"). Running is a good thing, even though about 30% of runners get injured every year. When the popularity of running exploded in the 1970's, new shoe companies like Nike tried to solve this problem and make the experience more comfortable by introducing shoes with lots of cushion, which in theory absorbed the shock of the ground impact. The cushy heel, however, caused runners to change their gait, creating a population of heel strikers. The heel absorbed the impact from the foot, but transferred the shock up the leg to the knees and back, creating an all new problem. Heel striking was never an option for runners before this new age of shoes, but now about 75% of the population are doing it. Runners are basically forced to heel strike in modern shoes, unless they excessively point their ankle downward, because the elevated heel catches the ground before the foot reaches a nearly flat clearance point. Barefoot running is gentler on the joints, as the weight should be centered over the hip upon impact, thus evenly distributing the force.
There's also a YouTube video which you can watch here: