Dogs bark for many reasons. When a dog barks excessively, it is important to determine the cause of the problem behavior. You may be able train him to bark less if you know what the reason is for his barking.
Bark collars use a negative stimulus to discourage barking. Some collars deliver an electric shock, others an ultrasonic noise or a spray. The chemical in the spray is citronella and seems to be effective at reducing the barking. These collars should not be the first choice for dealing with problem barking, however. Seek the advice of a good animal trainer or veterinary behaviorist to determine if a bark collar is an appropriate part of an overall behavioral modification protocol.