(Courtesy of Wikipedia)

There are countless "schools" or theories of natural horsemanship but the following ideas are common to most of them:

• Horses are social herd animals, evolved for social interaction and the ability to escape predators. The horse has a highly developed communication system practiced primarily through body language. It is possible for humans to learn to use body language to communicate with the horse. Horses use ear position, head position, speed of movement, threatening gestures, showing of teeth and swinging of hips, and many other gestures to communicate. They are quick to escalate a behavior if early warnings are not heeded. Similarly, in natural horsemanship, the handler or trainer uses body language along with other forms of gentle pressure with increasing escalation to get the horse to respond. 
• Most natural horsemanship practitioners agree that teaching through pain and fear does not result in the type of relationship that benefits both horse and handler. The object is for the horse to be calm and feel safe throughout the training process. A horse that feels calm and safe with his handler is quick to bond with that person, and the results can be remarkable.
• The human must be knowledgeable of the horse's natural instincts and communication system, and use this knowledge in their work with the horse.
• Like many other forms of horse training, operant conditioning through pressure and release are core concepts. The basic technique is to apply a pressure of some kind to the horse as a "cue" for an action and then release the pressure as soon as the horse responds, either by doing what was asked for, or by doing something that could be understood as a step towards the requested action, a "try". Timing is everything, as the horse learns not from the pressure itself, but rather from the release of that pressure. These techniques are based on the principle of reinforcement, rather than physical force, which most Natural Horsemanship practitioners avoid using whenever possible.
• Most Natural Horsemanship approaches emphasize the use of groundwork to establish boundaries and set up communication with the horse. This can include leading exercises, long reining and liberty work.
• As with all successful animal training methods, there is an emphasis on timing, feel and consistency from the handler.

Natural Horsemanship has become very popular in the past two decades and there are many books, videos, tapes, and websites available to interested equestrians. This philosophy has capitalized on the use of behavioral reinforcement to replace inhumane practices used in some methods of training, the ultimate goal of which is a calmer, happier and more willing partner in the horse.  
Natural Horsemanship avoids fear- and pain-based training methods. While natural and gentle methods of training have been around for millennia, dating to the advocacy of gentle methods by Xenophon in Ancient Greece. There have also been any number of techniques over the years that attempted to train a horse by breaking the horse's spirit, often forcing it to fight back and then be dominated or defeated. Natural Horsemanship advocates point out that by removing fear an individual gains trust from the horse. By not scaring and hurting the horse, the horse learns to work with people in a partnership verses as an adversary.
Some well-known trainers considered to be practitioners of Natural Horsemanship in the late twentieth century include: Tom and Bill Dorrance, Ray Hunt, John Lyons, Clinton Anderson, Buck Brannaman, Monty Roberts, and Pat Parelli.