A modern crop usually consists of a long shaft of fiberglass or cane or which is covered in leather, fabric, or similar material. The rod of a crop thickens at one end to form a handle, and terminates in a thin, flexible tress such as wound cord or a leather tongue, known as a keeper. The thin end is intended to make contact with the horse, whilst the keeper prevents the horse's skin from being marked. The handle may have a loop of leather to help secure the grip or a 'mushroom' on the end to prevent it from slipping through the rider's hand.
The length of a crop is designed to allow enough leverage for it to be accelerated rapidly with a controlled flick of the wrist without causing the rider balancing problems. Thus, a true crop is relatively short.
The term "whip" is a more common term that includes both riding crops as well as longer types of horse whips used for both riding and ground work. A whip is a little slower than a crop, mostly due to having slightly greater length and flexibility.
Crops are principally designed to back up the natural aids (leg, seat and voice) of a rider, but may also be used as a reprimand by more experienced riders, for example to discipline a horse for refusing a jump or other types of disobedience. However, care must be taken not to desensitize the animal to the stimulus.
The difference between a crop and a whip.
Dressage whip is a true whip, longer than a crop, (up to 43 inches, including lash or popper) for horse training, allowing a rider to touch the mount's side while keeping both hands on the reins.
Hunting whips are not for use on the horse, but have a "hook" at the end to use in opening and shutting gates without dismounting, as well as a long leather thong to keep the hounds from coming near the horse's legs, and possibly getting kicked.