A watchdog is a dog that is trained to alert their owners or caretakers when there is an intruder or potential threat, typically by barking loudly. They are typically passive and are not trained to actively attack an intruder. They are usually kept indoors as family pets and do not require extensive training.
A guard dog, on the other hand, is trained to attack and subdue an intruder. They are typically larger, more powerful breeds like German Shepherds or Rottweilers, and require extensive training in obedience and protection. They are used in a variety of settings, from law enforcement to personal protection for high-risk individuals or properties. They are usually kept outdoors and require special care and training to be safe and effective.