"Service dog" is a general term that can encompass all kinds of different canine professions. And service dogs are considered professionals
In fact, you are discouraged from petting them and treating them like pets when they are out with their owners. That's the time when the dog is "on the job" and should not be distracted.
What Do They Do?
Service dogs do what their name implies - provide a service. It may be a guide dog for a blind person or a dog that performs tasks for a physically disabled person. Overall, they help people to be more independent. Here are some of the jobs that service dogs do.
For people who are hard of hearing or who can't hear at all, a hearing dog can act a bit like "ears" for them. Dogs' hearing is already excellent, but hearing dogs are trained to alert their hard-of-hearing owners to sounds that need their attention. For instance, a hearing dog might alert a deaf mother that her baby is crying, or a hard-of-hearing man that his phone is ringing. We use our ears so often we don't think of the many things we would miss if we couldn't hear. A hearing dog is trained to catch those sounds we take for granted, and to act on them.
When a lot of people think of service dogs, guide dogs (or "seeing eye dogs") are often what comes to mind first. Despite a dog's red-green color-blindness, it can serve as the "eyes" for a blind human in most instances, leading the blind person around obstacles and preventing them from running into objects that are high enough for the dog to pass under, but not the human (such as a tree limb).
Many cities have auditory signals at street crossings to alleviate the problems associated with dogs' red-green color-blindness. Dogs can tell when signs and lights are lit or not lit, however.
Guide dogs allow blind people to independently get from place to place without the assistance of other people. This can be very liberating, and guide dogs often form close bonds with their owners.
People confined to a wheelchair or who otherwise have their mobility compromised may have a mobility service dog. This kind of dog helps reach things like light switches, items on shelves, or even shoes and household accessories. They might be trained to get a drink out of the refrigerator, or to bring their owners the telephone.
Simple tasks that the mobile among us take for granted, such as picking up something off the floor, are impossible for those with compromised mobility. A service dog can help them achieve a degree of independence and security.