According to Dr. Nicholas Dodman, dogs communicate through body language, facial expressions, and posturing.
Facial expressions aid in the communication of dogs with other dogs, humans, etc. For example, soft eyes, ears flopped, etc. express a relaxed mood. Dogs also communicate through odor. Since they have a very good sense of smell, they can usually tell a lot about other dogs from their smell.
As Dr. Dodman states in his article, "Dog to Dog Communication," "If dogs could talk, they'd categorize us, humans, as 'dumb animals.'
Dogs, Cats, Humans, and all animals communicate using a series of facial expressions, gestures and sounds. it's pretty universal when a dog growls and bares its teeth that it's really pissed. When it rolls over when you sometimes come to play with it shows an act of submission, since you, the family's "Pack Leader" is dominant. This also occurs in wolf packs. like marilyn said, a dog can learn a lot about other dogs through their smell.