The new wave of pet animal training focuses on positive or reward-based training only. The idea is to train your dog to perform certain desired behaviors rather than to punish unwanted behavior. For example, you would not train a dog to stop barking by punishing the unwanted behavior; instead you would reward the silence that inevitably follows.
Timing of rewards is critical. If a dog stops barking for three seconds and you have to reach into your pocket for a food treat and then walk toward the dog to deliver it, the moment may have already passed. Yet it is difficult to have primary rewards (food, water, toys) handy at all times, so how can this best be managed?
The answer is using a "secondary re-enforcer," like praise or a neutral cue that signals that the primary re-enforcer is due. In animal training, whistles and clicks have been used as secondary re-enforcers, though you can also use your voice. Clicks made by small plastic clickers ("frogs") are probably the best and most consistent way of marking the successful accomplishment of a behavior.
For more information on Clicker Training, please read Clicker Training Your Dog.