A clicker is a small box with a metal plate inside which when pressed makes a click. It is used to teach a dog a desired behaviour.
The clicker should be used like a camera - imagine pressing the shutter at the moment the dog is offering the desired behaviour (for example; sitting). Press the clicker the moment he sits. This is ALWAYS followed by a reward, normally food, and the dog soon associates the sound of the click with a reward and will repeat the behaviour.
A clicker is never used to attract the dog's attention, but is a wonderful way to teach a new behaviour. Once the dog has learned the behaviour then the clicker can be phased out and replaced with a word cue.
*Note clickers may vary in colour