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.

Types of guard dogs 
 Personal protection dogs are highly trained dogs who protect individuals or families against any threats. Personal protection dogs are trained to go anywhere with their owners and in many different situations. At home a guard dog may keep trespassers from entering. They have better hearing than humans and are usually aware of what is happening sooner. They act as a kind of home alarm. A dog's bark lets the owner know someone is there and alerts the trespasser the dog knows he is there. A guard dog can also provide protection should the intruder keep coming. Larger dogs make for better personal protection dogs. Some of the favorite breeds are German Shepherds, Boxers and Akitas. They are very trainable and look very intimidating to strangers.

 Livestock guard dogs also called livestock protection dogs are bred and designed for protecting livestock from predators. In most cases their ability to guard their herd is instinctive. The dog bonds with the herd from an early age. They are not usually used to herd or move a herd. Their only job is to guard the herd. Many shepherds seem to match the color of the guardian dog to that of their sheep. The theory is that it makes them less threatening to the sheep and harder to identify by predators. This may explain why so many Livestock protection dogs are white.

 Property guard dogs are trained to watch over property when the owner is not there. They are selected for being both loyal and intelligent. They often perform their job without someone giving them commands.

 Military guard dogs, sometimes called war dogs, are dogs used for military purposes. The US military requries they be 20 inches (51 centimeters) tall at the shoulders and cannot be more than five years old. They cannot be afraid of loud noises including gunfire. They are used for a wide variety of purposes and receive specialized training. Similar to Police dogs they may be used to patrol an area and are used against criminals. They may be used to detect harmful objects such as bombs, mines, harmful gasses and ammunition. Military dogs are trained to watch out for enemies and attack them. The US military used 32 breeds at the start of World War II but by 1944 the list was shortened to five breeds. These were the Belgian Shepherd, German Shepherd, Doberman Pinscher, Collie and Giant Schnauzer.

Breeds of Guard Dogs 
Different breeds of dogs are bred for different reasons, and are good at different things.  
The most common breeds used as personal protection and property dogs are:

Livestock guard dogs include:

Notes

References

Other websites 

 Dogs and Personal Security: An Introductory Guide
 Best Guard Dogs for Families

Working dogs