A daisy cutter is a horse that moves in a long, low action, without lifting his hooves off the ground more than is absolutely necessary.
Daisy cutting is a very efficient way of moving, so is commonly seen in horses bred for speed (Thoroughbred, Quarter Horses, etc). It's also desirable in show hunters and in pleasure horses. It's considered undesirable in dressage, where horses are expected to show elevation.
The daisy cutter moves his leg primarily from the shoulder, not the knee as might be seen in gaited horses and dressage horses.