A lot of people believe that if a dog is wagging his tail, he is friendly and approachable. This is flatly incorrect and it gets a lot of people bitten.
As with any other body language, we must look at how the dog is wagging and at the rest of his body to get a better idea of his mood. Different tail wags can indicate friendliness, happiness, uncertainty, fear, and threat.
This topic has been well covered elsewhere, but I wanted to share a marvelous bit of video a friend took of one of her dogs. This little Jack Russell (her name is No. 5, as you can hear one of her owners saying in the video) is ferociously resource guarding a trash can. As you can see, her owner is easily able to call her off of her little guarding project to distract her before removing the interesting item from the trash. It’s amusing that this dog is willing to (apparently) lay down her life to protect her access to the empty gummy bears bag, but the reason I am posting this is for her body language. The guarding is unmistakeable, but look at that tail! Wagging madly the whole time! (It’s wagging high, stiff and fast, which is a typical “threat” wag.)