Most wyverns have broad, membranous wings in place of true arms. They lack the six-limbed structures of dragons; dragons have four true legs in addition to wings. Wyverns are usually not sentient creatures, but they're quite intelligent animals. They are masterful flyers, occupying a position in the food chain just above most birds of prey.
Smaller varieties are quite trainable and are often used as familiars by mages. Some have theorized that they have about the same ability to learn and recognize commands as dogs, pigs, or sea otters.