Almost all animals use body language of some shape or form to communicate with each other. Some mammals rely on body language more than others. Humans communicate with body language on a daily basis without even realizing it at times. Dogs are no exception. A dogs unique body language can tell a lot about what it's feeling and thinking. After thousands of years living with humans dogs have developed the ability to be able to read our body language better than we may think. We could at least do the same for them. A huge percentage of people live in a household with dogs, unfortunately many of these committed dog owners wrongly think that they have their pooches body language down. To them, every tail wag means a happy dog and every curled lip means a dog about to bite. But dog body language is so much more complex than this, and just taking a few minutes to learn and educate people about what a dog is really trying to tell you could very well prevent a dog attack.
To be able to read what a dog is saying through body language, you are going to have to get out of the habit of just reading one aspect of the body, such as the tail or ears. Instead it is a combination of all of these subtle cues that will give you the bigger picture of the intentions of that dog. For example a picture of an exited dog vs. a tense dog ready to bite may be identical except for for the level of relaxation that dog has.I have a German Short-haired Pointer who curls up his lips and shows his teeth in a big happy smile every time he comes home, it's just his way of expressing his excitement. Someone who did not previously know this and limited themselves to just looking at his teeth might think he were ready to bite, but the enthusiastically wagging tail and loose body language would otherwise tell you that he is simply excited to see you.
The main body cues to look for are going to be in the eyes, mouth, ears, tail, and how relaxed or tense the entire body is. Ears back may mean fear, aggression or just listening to something behind them, depending on the context. If the ears are back and the body is cowering, it's safe to say the dog is scared. If the body is rigid and upright the dog is probably being aggressive or defensive.
The eyes can tell a lot as well. the whites of the eyes in a dog you otherwise wouldn't, this is a sign of fear. Relaxed and happy looking eyes tell you the dog is just that.
Most people know to stay away from a dog who is showing teeth and growling, but what is less known is that showing teeth can also be a sign of submission or in some cases like mine, excitement.
The tail seems to be the part of the body that has the most misconception. An important thing to remember, a wagging tail does not always mean a friendly dog. Dogs will often wag their tail if agitated, though you can usually tell the difference between an agitated wag and a happy wag. The full tail and sometimes full body, sweeping motion is a happy dog, while a quick stiff wag with a raised tail is a tense dog who could act out aggressively if provoked.
A good way to determine what the tail says is to look how it is held. If it is raised, the dog is on alert and could be either aggressive, or just watchful and anticipating. A tail held straight out is usually pretty neutral, a relaxed tail is a relaxed dog, and a tail between the legs could mean a fearful or submissive dogs.
As a side not, fear based aggression is the most common form of aggression. A dog is more likely to bite you if he feels cornered and scared rather than defensive and aggressive.
There is one last thing to look for in some situations, such as when a loose dog comes charging towards you and you have to decide whether to get ready to defend aggression or ignore the dog, and this is how intense the dog seems to be. If a dog is charging towards you barking, chances are he is not going to bark because he is expending energy barking. A dog that is charging full speed at you while focusing intensely on you, not barking, possibly snarling or maybe even completely silent, is one that could very possibly bite if not handled right.
This is a general guide of dog body language, and while there are a few universal ques to help tell you the motivation behind a dogs actions, every dog is different and acts in its own unique way to different stimuli.