Some boots are:
Brushing boots or splint boots, that are used to prevent injuries caused when one leg knocks against the other.
Tendon boots are used to protect the the tendons which run down the back of the horse's lower leg, so they are usually open-fronted.
Knee boots prevent damage to the horse's knees when jumping. They are not suitable for roadwork or travelling.
Coronet ring is fitted to one of a pair of legs to prevent brushing injuries.
Overreach boots or bell boots prevent overreach injuries. An overreach is caused by the toe of the hind striking the heel or the coronet of the front.
Travel boots are simple square padded wraps that are effective in protecting the lower leg whilst the horse is travelling.