Mine bark at everything: people, dogs, cats, rabbits, birds, cars driving down the street, the windsock on the porch, each other, and, I think, just general barking because they want me to come and see what they're barking at (nothing). My dogs are pretty big and they sound ferocious but I'm sure if somebody were actually there, they'd only get kissed to death.
In spite of their loud barking, I had the electrician come one day when I wasn't home and I told him if the dogs bothered him to lock them in the bath room. I talked to him later and asked him if he'd had any trouble with the dogs. His answer was, "What dogs?" Apparently they only bark when I'm home. The rest of the time they go upstairs and cower under the bed till I come home. So much for keeping me safe. LOL