Both guard dogs and watchdogs bark loudly to alert their owners of an intruder’s presence and to scare away the intruder. The watch dog’s function ends here; a guard dog is capable of attacking, or restraining the intruder.

Livestock guardian dogs are often large enough, 100-200 lbs and strong enough to attack and drive away livestock predators such as wolves. Some smaller breeds (such as Keeshond and Tibetan Terrier) are excellent watchdogs but not guard dogs because they bark loudly to alert their masters of intruders but are physically small and not given to assertive behavior. 

Guardian breeds will bark to alert their master and to warn off an approaching animal, or human threat, prior to their interception of the trespasser.