Dogs have a wide range of vocalizations designed to communicate with the creatures around them. Humans can often misinterpret these sounds, so it’s important for dog owners to have a good idea of the intentions behind your dog’s bark or whine.
Barking is generally the most common canine vocalization. Barking is common at the approach of a person, dog, or other animal in order to to warn the animal, ask for play, or signal distress. Excessive barking when left alone can be a sign of separation anxiety.
Some dog breeds have a propensity for howling. Wolves howl with their pack in order to connect before hunting. Even if a dog does not hunt, it may howl to seek connection with another creature.
Growling can be used in play or as a defense signal. Many dogs will mock growl during a game of tug-of-war. Growling is also the signal of aggression and dominance, however, generally signaling that the dog wants to get their own way.
Source:
http://www.canismajor.com/dog/cancom.html
