General:

Horses, like other animals living in groups, are quick to establish a chain of command which lowers the number of fights over food, water, and other horses. Communicating with each other through social signals like pinned-back ears or bared teeth, the social hierarchy is soon established and a lower-ranking horse almost always gives way to a higher-ranking horse without a fight. Unlike creatures who communicate through many different sounds, the horse's number of different vocalizations is actually quite limited compared to other animals.
Wild herds consist of a stallion and his band of mares. The stallion is the top authority, who locates open places to graze for his herd, finds water, and chooses where to go. Second in command is the lead mare, who keeps the members of the herd under control, or takes over as the head when the stallion goes off on his own. She is the smartest mare, the one that all the other horses respect.
The lead mare takes care of the occasional disputes and quarrels between members of the herd. For example, if a young horse is bothering one of the mares, the lead mare will even drive him away from the herd if the problem becomes serious. This is considered a very harsh punishment because predators are far more likely to go after a lone horse than a herd. The mare is very strong and dominant, yet wise. She answers only to the head stallion, who entrusts her with the responsibility of keeping the herd together.

Touch:

Horses rely heavily on touch to communicate with other horses. Whiskers are used to touch and feel objects, mainly food. Mutual grooming between horses is a form of ritual bonding within herds. Horses touch and nuzzle each other as a sign of friendship and acceptance. Physical contact with other horses is vital to the mental well-being of your horse.

Smell:

A horse's sense of smell is far more acute than that of a human being. It is able to recognize friends by smell alone. Horses are also able to smell the fear scent that is given off by others.

Body and facial language:

Body and facial language are primary communication tools for horses, with fear, curiosity and aggression often being displayed in this manner. In the case of fear, horses often flare their nostrils [Facial expressions communicate a lot about horse behavior] and flatten their ears against their heads. Aggression, which can produce similar facial expressions, on the other hand, should not be mistaken for fear. Such expressions of alarm would normally be accompanied by a clamped down tail and in more defensive stages, by an open mouth and a leg being raised.

Oral Communication:

Physical expressions are often accompanied by oral communication. Snorting , associated with a long intense stare, is a horse's way of alerting its peers to the possibility of danger, or of asking other herd members whether they are under threat. Determining whether an intruder is a friend or foe is normally manifested by a large blow through the nostrils, similar to a snort. Nickering is a friendly form of communication between horses that they recognize as friends.
More aggressive approaches are displayed as a squeal or a scream, the latter being the more serious and generally indicative of a fight that is about to commence. A squeal is generally deemed to be a warning sign.
The neigh is the loudest and most commonly used form of oral communication and is rarely an indicator of fear. A horse normally uses a neigh to locate other horses or to announce its own presence.
PLEASE try and keep somewhere within these. If you are a low ranking mare you DO NOT pester higher ranked mares/the lead stallion. They can punish you as they see fit ;)