A sniffer dog, or detection dog is typically trained to sniff out things like hidden drugs and other contraband by first learning how to sniff out its favorite toy, laced with the substance the dog will eventually make a living out of looking for. The toy used depends on the dog being trained.
Training a detection dog is a long process, but it is filled with play and positive reinforcement for the dog. Dogs first have to learn to play fetch or tug-of-war, two games most dogs love. They are typically taught commands like "search" or "find." Training requires plenty of repetition.
Training is started with a toy, then the toy is laced with the substance the dog will be expected to find. Eventually, the toy is removed from the equation and the dog will learn either passive or aggressive scent detection. Aggressive detection includes pawing and barking at the object while passive detection involves the dog sitting when it smells something illegal.