Wagging the tail is an emotional language that dogs speak. If the tail sets are not wagging, it shows that the dog is relaxed. The relaxed position of tails may differ between breeds of dogs where their tail hangs down to their heels while some have their tails curled upwards.
If you see your dog holding its tail lower than its natural position, then the dog is scared or nervous. If your dog is curious it holds its tail straight.