Andrea Rosebrock 
We’re so used to seeing a dog’s wagging tail whenever we approach our own pooches and we know that it means our dog is happy to see us but it seems there’s more to a wag than we thought.
Our pets have their own ways of letting us know what they want or need, like or dislike, through their spoken barks or meows, but mostly through their body language. And although our pets use their whole bodies to communicate, their tails are such a visual indicator of what’s going on in their heads, it’s worth focusing on.
It’s interesting to note that the tail’s primary purpose is not to communicate, but for survival in the wild. For dogs it’s used for agility, to help them change direction quickly when chasing prey. For cats, it’s for balance when walking out on a limb, and more importantly, to help them flip over in the air to land on their feet if they fall off that limb.
But our domesticated pets use their tails mostly to communicate with their owners and others of their species.
We all know the happy wagging of the tail. It wags when we come home. It wags when a treat appears. It wags when the leash comes out and he knows it’s time for a walk. Therefore, we read the wagging tail as “I’m happy!”
However, a wagging tail actually means “I’m excited!” And although that usually goes with being happy and friendly, it can also mean “riled up.” So when you, or your children, encounter a strange dog, a wagging tail alone doesn’t necessarily mean it’s ok to approach. It just means he’s excited by your presence, and he’s not all that afraid of you.
If he’s a stray dog that hasn’t been socialized, that excitement could turn to anti-social behavior, such as an overly rough greeting or even a bite. (Yahoo Pets)
Who’s Saying What