There are many similarities and differences between dogs and humans. One of the main characteristics is that both humans and dogs are social. This is one of main traits that helped advance the domestication process. Dogs also have a social structure. There's the alpha and the omega. Which are commonly known as the top male and female of the pack.
It's from this structure that a myriad of unique vocalizations, gestures and actions have evolved. Because dogs are direct descendants of wolves, almost all dogs have the exact same ways communicating, acting and reacting. This is so well ingrained in the canine species that even anyone who has never had a dog can tell when a dog is mad or afraid. The body language of both of those characteristics is easily recognized by anyone even if they aren't "into dogs".
Perhaps the greatest advantage we have is that both dog and human body language are very similar. Both dogs and humans tend to stand very upright and make direct eye contact when they're mad. Likewise if both are timid they tend to have a crouched, sideways body posture. When we see someone dear to us we tend to greet them joyously (hugs, hand shakes, kisses, etc.), and dogs show their joy by wagging their tail vigorously, flattening their ears and many jump up, turn in circles, etc, etc. It's these similarities that have contributed to the saying "man's best friend".
On the other hand there are also many body postures that are completely different than ours. These postures have a huge array of different meanings that can range from extreme submission to extreme dominance and everything in-between. If/when we're able to gap the bridge of knowing the meaning of these postures, then we'll be able to truly understand and communicate better with our dogs. The best trainers in the world are usually those who have bridged that gap and have developed what we know in dog training circles as "reading a dog". As in all, it's not as simple as it looks, if it were everyone would be trainer!
A pup may have a precise reaction to a stimulus yet his reaction to that same stimulus may differ completely once he's grown. For example; a human aggressive 3 month old pup doesn't necessarily mean it'll be a HA dog once grown. Anything from insecurity, genes to improper socialization could be the culprit. And like this there are many other examples.
All dogs have different amount and degrees of natural instincts that have been well deciphered and categorized into named drives. When you look at a dog evaluation for defense, you'll see the agitator using ample body language to entice a reaction in that dog. The agitator may crouch down, make odd noises and mix up his "approach" until he gets a proper response from the dog. From there the agitator can build on to get the training done. Depending on the dogs' temperament and levels of his different drives, he'll react accordingly to the agitator's moves.
The same way we learn to read dogs, dogs also learn how to read us. Our body language is so strong that dogs can read us like a book. Try making a dog come to you when you're steaming mad. I'm sure everyone at one time or another will find themselves loosing their temper on their dogs' wrong doing and they'll shout to their dog; "Come" in a harsh tone. Analyze your dogs' reaction. Is the reaction of your dog the same as when you call him when you're not angry? Most likely, not. Dogs will also learn how to read you and know when you feel depressed, anxious, frightened, etc.
How many of you had "unwelcome visitors" come by and your dog also treats them "unwelcomely"? I've heard many people say that their dog can't stand certain people even though those people have never done anything bad to the dog. What they don't realize is that as a member of your pack, your dog will dislike most anyone you dislike too. It reminds me of a funny incident some time ago. We were training at a park and a friend of mine brought what I can say is a foe of mine. I really disliked this person. The dog I had at that time was my retired French Ring champion. In the sport of French Ring there's an exercise called the Object Guard. In this exercise you take a basket to the middle of the field and tell your dog to guard it, then walk away into a blind where the dog can't see you. Then the decoy will come out and try to "steal" the basket away from the dog. The dog must bite the decoy in every attempt yet must let go and return the basket no more than 5 seconds once he's bitten the decoy.
During that specific training day in the park my wife was sitting down on the grass playing with my dog and talking to some other people there. Mainly to the wives of the people I was training their dogs. It so happened that we needed a training tool that my friend had in his car. He told my foe to get it. Suddenly my wife yelled at me. When I looked she said that she couldn't get our dog off of her. As looked at him, I saw he was protecting her from my "little foe" who had to walk by closer and closer to her as he came back because of the fence. Our dog was doing an Object Guard with her! That's when I said to my foe to stop walking and take the longer route if he didn't want to get bit. Lol
Body language can and has been used extensively in dog training and trialing. Commands, praise and corrections can be taught to dogs and then minimized to simple gestures that can't be picked up by anyone unless you're advised and aware of it. When I competed, specially in French Ring where the order of the exercises are randomly picked just before you compete, I had a series of signals that let my dog know exactly which exercise was coming up, a correction to tighten up, a huge euphoric praise, etc. All these things by a simple gesture, noise or touch on a specific part of the dog's head or body. Dogs are truly one of God's greatest creations. Happy training!