Structure of the Mouth

The adult male horse has a total of 40 teeth, made up of 24 molars, 12 incisors, and four tushes. They may also have up to four wolf teeth. Adult female horses do not usually grow tushes, and so have a total of 36teeth, plus possibly up to four wolf teeth.

Horses are naturally grazing and chewing animals, and as they grind up their food, they are continually wearing down their teeth. By around five years old or so, a horse's permanent teeth will have grown to their full length, and from then on, the teeth are gradually pushed up through the gums to compensate for the continual wearing down of the crown.

As the teeth ire slowly pushed up from the gums, different sections come into view, and this is how we can tell ipproximately how old the horse is.
Another point to bear in mind is that the horse's top jaw is wider than the bottom jaw.

This means that over time as the horse is grinding up his food, die teeth wear out unevenly. Sharp edges develop on die outside edge of the molars of the top jaw, and the inside edge of the bottom jaw, and these cut into the cheeks and cause pain and discomfort, which is why it is very important to have horses teeth rasped.

Horses need to have their teeth 'rasped' approximately twice a year, and this involves either a veterinary surgeon or equine dentist checking the teeth and rasping off any sharp points that may have developed on them.