How many times have you seen a wagging tail and thought "Oh, that's a friendly dog?" I hear people say this all the time but looks can be deceiving. Tail wagging can be a sign of so many things that you need to see what the rest of the dogs body is doing first. The tail is very good at demonstrating the dogs state but if you go by tail alone you may get a surprise.
Tail position is a biggy, is it held high, midrange, down or tucked. In all of these positions a dogs tail can be wagging. Wag speed means differnt things as well. Very fast wagging is a sign of excitement; and excitement is not always a good thing. Nice flag type wagging with the tail held between upright and halfway is usually the best wag. This typically means that the dog is happy or content.
Low tail wagging although submissive in signal can be fearful. A fearful dog can be dangerous and many people receive a bite very unexpectantly. Often you think of a fearful dog as non threatening because they are afraid. This is the case sometimes but other times a dog will lash out because they are afraid; watch the rest of the body.
A wagging tail is only one sign to watch for behavior in dogs, the other two important signals are body language and the eyes.