Simply put, a growl is a sound signal that warns another animal to keep its distance. In many cases it is part of demonstrating dominance, dominant-aggressive and aggressive behaviour – whether natural or unnatural – and can precede an attack.
Anyhow, a dog in a good mood can growl as a call to play; also, they can loudly growl during playing.
To estimate a dog's intention, you have to take in notice the whole body language as well as some parts of the body. Only then it is possible to correctly asses the intention of the dog.
1. Deep, yet quiet growling, if a dog is in high body position with dominant attitude, represents a serious warning, which, if the behaviour that causes it continues, can turn into attack. If a dog growled in such manner and suddenly stopped without changing body posture or any part of the body for that matter, attack is very much evident. Such sudden silence means that the threat was not accepted and attack is necessary – like the calm before the storm. Example could be encounter of two dominant dogs where each wishes to showcase its dominance. Common prelude is greeting dance.
2. Quiet but a bit higher-toned growl. A dog is giving a warning as it is not eager to start a fight. However, it depends more on the other dog's reaction. If it does not back off, it will get into a fight because it is not scared. If they have owners, much depends on the owners' reactions.
3. Growl that turns into a bark forms a unit that a dog repeats. It means that the dog is not as confident. However, it will attack if it is inevitable.
4. Growl and bark at the same time. It is not feeling confident but is truly using this signal as a threat and will fight back.
5. A growl that changes key from lower to mid-higher while changing the rhythm – wavy growl. It can add to it a short bark. It can be a sign of a protest for having to stay still because the dog is not approving of it.
6. Intense, very loud growl of lower note, while teeth are completely hidden can be expression of content during play time.