Dog body language

The other most obvious of the three means communications that dogs use is body language. This form of communication is understood by paying attention both to the postures of the dog and its facial expressions. Dogs typically use body language (and facial expressions) to express these four messages: domination, aggression, fear and submission.

Facial expressions: One of the primary ways a dog shows how it feels and what its intentions are is through its facial expressions. A relaxed and comfortable dog will have no specific facial expression. Its eyes will be half shut and its ears will be hanging limp against the sides of its head. If something catches the dog's attention it will widen its eyes stretch out its neck and turn its ears in the directions of whatever it is interested in.
In the moment a dog's interests transforms into "alarm" it stands up to control it's surroundings in case it needs to defend it's territory, it's owner, it's puppy's or it's bones. The dog's facial expression will be aggressive indicating threat and the will dog growl by showing its teeth trying to intimidate its challenger, the hair of the dogs front shoulders bristles, and it will often times bark furiously.
When a dog is ready to attack, it shows it's teeth even more and its growl becomes deep and it puts it's ears back.
As the dog's tension increases, all of its expressions transform: the eyes widen completely, it fully bares its teeth and all of the hair along its back stands up and bristles. If these warning signs are not enough to deter there would be threat, then this body language is quickly followed by physical action to deal with the perceived threat. On the other hand a submissive dog has its head, tail and ears down and lets its back hang down in such a way that its genitals and sensitive areas are available to be examined by the dominate dog (or human in some cases). The submissive dog will sometimes urinate as a further sign of its submission.