When a dog's tail hangs lowly or is tucked between his legs, it exhibits nervousness or a sign of submission. Some dogs' tails, such as the greyhound's, naturally hang lowly, showing that the dog is relaxed and friendly.
When a dog moves his tail into an upright, rigid position, it is a sign of alertness. This occurs when something startles or arouses the dog. While some tail-wagging indicates happiness, stiff tail-wagging can be a sign of aggression against an intruder. A tail tucked up against a dog's belly is a sign of fear or extreme submission.
In addition to their tails, dogs also express themselves with their ears, eyes and other body language.