|Because the dog's ancestors were pack animals, they needed to develop a way to communicate with each other. They use many different types of body postures and vocalizations as a type of language. The tail is extremely expressive and an important part of the body posture.
A friendly dog will communicate his intentions through loosely wagging his tail. But, a wagging tail does not always indicate friendliness. Dominant or aggressive dogs may wag their tail, but the tail is held high and maybe just the tip of the tail is moving rapidly. A playful dog will also hold his tail high, but it will be wagging greatly from side to side. A dog who holds his tail low and wags it stiffly may be indicating he is submissive or afraid.
The tail is just one part of the dog, also look at his body, head position, ears, eyes, and hackles to help you understand what the dog may be trying to express.