Dogs bark to communicate with other dogs and with humans. Dogs are descendants of wolves, which are social animals that live in packs, and they share many of the behaviors that define the complex relationships that exist within such animal groups. Few domestic dogs live together in packs (though they often consider their human family their group), but they still use complicated behaviors that involve smell, sight, and hearing to communicate.
A dog has many scent-producing glands that it uses to communicate. The scent that a dog leaves behind (in its urine, feces, and paw prints) can reveal its sex, age, and even its mood to other dogs that come sniffing by. A dog uses its posture, facial expression, and ear and tail position to communicate with other dogs, too. And it uses its voice to communicate by whining, growling, howling, or barking.
A dog usually whines or whimpers when it is in distress: when it is hungry, cold, or in pain. Growls indicate that a dog is angry and ready to fight. Howls and barks usually show excitement. In the wild, wolves and other canines use howling and barking to call together the pack for a hunt, for feeding, or to warn against danger.
When wolves were tamed, or domesticated (between 12,000 and 14,000 years ago), the less dangerous ones were kept to act as guard dogs, to help in hunting, or to herd other domesticated animals. Barking was a useful trait, a handy alarm system that let their human masters know of approaching intruders, prey, or predators. This desirable trait was bred into new dogs. This means that the owners would arrange for two dogs who barked a lot to mate, producing a litter of pups with a tendency to bark often. Then those pups would be bred with other pups who liked to bark, and so on. Over time, domesticated dogs came to use their barks to communicate with their owners. Today, your pet dog barks when it's excited, needs attention, or wants something. It uses its bark to communicate much more than it would if it were living in the wild.
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