A guard dog, sometimes called an attack dog, is a dog that is trained to watch for and guard against people or animals who should not be there. Their main job is to keep their owner and the owner's property safe from danger. A guard dog is not the same as a "watchdog". A watchdog will let its owner know that a stranger is coming by barking. Guard dogs, however, may be trained to attack if their owner tells them to. A guard dog may also be trained to attack if a stranger comes too close. If trained properly, they will give a warning before attacking. A different type of guard dog is a livestock guard dog. Certain breeds of dogs are natural guardians of livestock. Their job is to stay with and guard a flock or herd against predators.