Well, there are different types of growls and different types of tail wags which all mean different thing. Not understanding the difference can get you either bitten or resenting your dog.
A growl can be a warning, but some dogs also growl during a play etc. Quiet growl the comes from the chest is clearly a warning growl. Throaty growls can be part of play or posturing.
Meaning of a tail wag depends on the speed and tail position. Speed of the wag simply means level of excitement or agitation.
Position means the state the dogs is in - aggressive, submissive or relaxed.
Flagged tail (tail straight up) is a warning. Could communicate dominance or agitation. Generally tail up wagging slowly is dominance, tail up and wagging quickly is highly agitated dog who can bite.
Tail in a relaxed position can wag slowly or quickly, the dog is happy with different degree of excitement.
Tail tucked down means submissive or fearful.
There are other signs such as position of the ears, type of stare.
I might be best of learning about dog body language first, so you can really understand what is your dog trying to say.
The situation in which this happens might also give you a clue. (e.g. trying to solicit play or attention, having a bone or chew toy)
Depending on what the dog is communicating, different approach to the situation is needed.
You do need to learn about it or get somebody to observe it and explain it to you.
it means the dog dose not mean to be mean but is only playing fighting
my old dogs did the same thing before they died and my new dog does it 2. It's completly common. It means they are trying to communicate and it also means that they want to play or go outside. This is what it always meant with my dogs they always do it it's only threatining when they try to bite.
it means that he is nervous and is contemplating whether or not to bite you. treat it like a case of fear aggression
Wagging his tail just means he's excited, it doesn't mean he's happy or being friendly. Dogs are generally very excited when they try to start a fight, for example.
What happens before the old dog growls at the new one? Is the tail held high over the dog's back or down low?
What is the dog growling about? Thats what I need to know first. Is he growling about you taking his food or you playing with him or what? If he is a puppy and you are playing with him then you dont have to worry about anything. That is normal for puppies to do that. But like you said, if its a huge dog and he is growling over food, then you need to do something quick. He is trying to tell you guys that he wants to be boss and you cant make him do anything. He tells YOU what to do. Then that is bad. I would start working with him. Teach him tricks or somthing. Work his brain. And when you give him his food make him sit down and wait till you put his food down and THEN he can eat. You make sure he waites for YOU to let him eat or let him go some where. Teach him the sit and stay command. It helps him better learn that YOU are in command.
A wagging tail does not always mean friendly. It may also be a sign of excitement or stimulation. Does her old dog bare its teeth or has its hackles up?
Well there's no problem, if the dog sticks up his butt and ways his tails while his head is low then that means he wants to play, another reason could be that the dog is nervous about meeting new people so he does that to protect himself but in a playful way.