There is a lot of information contained in the movement of your dog’s tail, which is why tail wags can mean so many things. The speed, amplitude, and looseness of the tail are all informative when assessing a tail wag, as is the height of the tail. The complexity of the tail wag is immense, and while many people unfamiliar with dog behavior think that a tail wag means a dog is friendly and happy, dog savvy people all know that this is an inaccurate and oversimplified interpretation of the tail wag.
However, there is one particular type of tail wag that means what most people think a tail wag means—that the dog is happy or friendly (or both). That type of wag is called a “circle wag” or, less commonly, the “propeller wag” or “helicopter tail”. Both terms are quite descriptive and mean that the tail is rotating at the base and that the tip is moving in a broad circle. I love to see circle wags because they tend to indicate such friendly, happy dogs.
via Whisker Therapy