Read your Dog's Body Language

First, check out the tail. The way it wags is the most recognizable kind of body language and easiest to interpret. Here are some motions and meanings:
Gentle waving motion-—relaxed and friendly. "Hey, what's up, good to see you."
Up at attention, vibrating or fast wagging—-excitement, tension; usually seen around other animals. "Are we gonna play now or fight or what?"
Tucked under—-submissive, fearful. "I'm cool. Please don't hurt me."
Tail wags in a big circle--greeting. "Good to see you, how's tricks?"

Ears are a good indicator too:
Relaxed--at rest, everything's cool.
Pinned back--feeling fearful or submissive.
Pricked forward--on alert, could be aggressive or expecting aggression. Also, "Is that a squirrel I heard in that tree over there?"

You've heard that the eyes are the windows to the souls? Well, that definitely holds true for dogs. You’ve probably seen them stare each other down. They’re each trying to assert their dominance. Usually, the bigger dog wins and the smaller dog will acknowledge this by glancing away or never even giving full eye contact in the first place. The alpha dog's like, "Are you cool?" The submissive dog’s like, “Yeah I’m cool, boss.”

By the way, you should always look straight at your dog when you are correcting a bad behavior. This reinforces the fact that you, indeed, are the alpha. And don't you forget it, dog.

If the hair on a dog’s back is raised, it's a sign of nervousness or feeling threatened. “Okay, there’s something going on here, not sure if it’s good or bad yet.” There’s even a fancy term for this—piloerection. If he's in the presence of other dogs, watch out, there may be a fight a'comin...

Now for the oral part of the lesson. If your dog's mouth is slightly open and relaxed--that's the smile we all like to see.

Yawning can mean "yeah, it's WAY past my nap time," but it could also be a sign of uneasiness and stress. For humans, too, by the way.

The one you really have to watch for, though, is when they pull their lips back to show their teeth. This means, "You threatening me? Are you... threatening me?! See these teeth? I know how to use 'em." When you see your dog do this, immediately scold them, which redirects their attention. Time to show them who's boss and it's you.

Finally, the dog's actual body position will tell you a lot. Most of us are familiar with the down dog--tush up, front down. May be accompanied by a short bark. Means “let’s play! Want to, huh, want to?”
The opposite position, front high, back low means he's scared or excited.
Standing erect and at attention says "I’m a confident dog who’s got it all going on."
Laying on the back, showing the stomach, is the ultimate submissive position. "Do what you will to me, but please, be gentle."
Once you learn how to gauge your dog's reactions, you'll be much better able to stop any aggression before it starts. And when you're ready to play, try a "down dog." It's like you're speakin' their language!