Both guard dogs and watch dogs bark to alert their owners of an intruder's presence. The barking is also an attempt at scaring away the intruder. The watch dog's function ends here, while a guard dog might be trained to restrain or attack the intruder. For example, livestock guardian dogs are often large enough and strong enough to attack and drive away predators such as wolves. In cases where intruders are more likely to be human, guard dogs are sometimes trained simply to restrain an intruder with his owner's guidance, as in Schutzhund or K9 Pro Sports training. Occasionally, guard dogs are trained to attack human intruder(s), though this practice might be locally illegal. Some breeds (such as Keeshonden) are excellent watchdogs but not so excellent guard dogs, as they bark loudly to alert their masters of intruders but are not given to attack behavior.
Many of the now prominent guardian breeds such as Rottweilers started as farm dog types but then developed over many years into guard breeds. Some breeds, such as the Weimaraner and Rhodesian Ridgeback, were originally bred for hunting, but their large intimidating look and territorial instincts have helped them evolve into guard dogs in today's society. Others like Dobermans were specifically bred as guard dogs. Many of the below breeds have a greater amount of molossoid or mastiff DNA. This is confirmed by a DNA study done on >270 pure breed dogs. The study suggests that in the distant past thirteen ancient breeds broke off early on after which a group of mastiff style dogs were developed. These dogs are grouped with the mastiffs Great Dane and Irish wolfhound. The Irish wolfhound (a traditional hunting guardian breed) has evidence that its population was nearly wiped out 200 years ago and its existing members descend from a very small group of dogs and thus it is a rebuilt breed, but evidently out of somewhat different building blocks. Some people think that the Irish wolfhound contributed to the Great Dane.
The St. Bernard is also somewhat different from the mastiffs and is not in the molossoid group; however, the Swiss mountain dogs are. The Moscow Watchdog thus likely inherits its guarding ability not from the St. Bernard but from its other major contributor, the Caucasian Ovcharka (no evidence other than speculation and the known breed characteristics). Great Danes are odd because they are not in the molossoid group and traditionally behaved as though they should be.
The original Saint Bernard was used for alpine rescue in the Saint Bernard Pass by the monks. An avalanche killed off many of the dogs used for breeding. The Saint Bernards had to be bred with larger dogs such as mastiffs which gave it the large size and the guard dog instinct which the Saint Bernard has today.
It is claimed that female dogs tend to make better personal guardians than males, due to maternal instincts, but males are considered better for guarding property because of their greater territorial instinct.