I found a nice explanation to your question on the internet. Please look at the following link and you can refer to it anytime here.

The text says that dogs bark to say that there is something strange happening and to be alert. In the wild the barking tells puppies to hide and calls the adults over for action. At home, our dog may bark at us or at a burglar. The barking signals that something is happening; once the new arrival has been identified, either a greeting takes place or an attack.

A fearless dog that is intent on attacking is silent. It doesn't waste time barking, that is, sounding the alarm. It just rushes over and bites. On the other hand, a dog that wants to flee instead will also be silent as it runs away.

A more common occurrence is when a dog is not quite sure -- it is feeling a little fear while it thinks about attacking. This dog will snarl and retract its lips to reveal its teeth. It is the tinge of fear that converts the silent attack into a snarling one. The urge to attack is still strong, however, so this is not a dog to be trifled with!

If the dog is more fearful, it will alternate barking with growling. If fear gains the upper hand, the growling will stop, replaced by loud barking, until either the threatening situation goes away or reinforcements arrive.

Apparently, barking was improved during the process of domesticating the dog. Wolves bark, but not as loudly as dogs, although they can learn to bark more loudly if they live around domestic dogs. It seems likely that humans bred the louder puppies to create better guard dogs. All dogs bark, except the Basenji, which was bred thousands of years ago as a silent hunting dog.