You can learn a lot about a dog's tail! the below pictures detail the different cues you can take from a dogs tail:
This dog is alert and focused, revealing confidence. It is common of breeds similar to this one to wear their tails very high and confidently.
This dog is relaxed. The tail is not tucked or insecure appearing, but neither is it high and stiff with tension or dominance.
This dog looks quite fearful, judging by the way his tail is tucked, his posture hunched, and the hesitancy he is showing through the position of his head and ears.
Tail is right in the middle, this is ideal! A low tail can show insecurity and a high tail dominance, but a tail in the middle is generally just right
The Shiba Inu is one of many breeds that naturally has a very high and curled tail. Keeshonds, bichon frise, and Chow Chows also have naturally high tails. It is still possible for such dogs to tuck their tails however, and if you pay close attention you can still see a difference in the height that they carry their tail.
Rottweilers, Dobermans, some spaniels, some pointers, and many other breeds customarily have their tails docked. Generally, it is still possible to gain information on the dog's energy and attitude from their tail even when it is docked.
This picture is great because it shows confidence and dominance in the dog on the right, and fear/submission in the dog on the left. Notice the difference in posture, head, ears, tails, and chests in the two dogs.
One of the most common misconceptions about dog is that the tail wagging means happy. Tail wagging means EXCITEMENT! This little dog is excited about whatever it sees off the side.
Similarly, I would guess that these three dogs are excited to get a treat.
These dogs are alert and warning, notice how their tails are high and wagging. I would guess that they are telling a stranger not to come too close because this is their property.
Aggressive dogs will often wag their tails when they are warning, attacking, or fighting. This dog is over excited, dominant, and potentially dangerous.
Last but not least: the tails of dogs at work! This is the ideal way for your dog to look when he/she is working: right in the middle. notice how none of these dogs has a high or low tail, they are all focus and full of purpose.