The Flash head depicted above is called a SpeedLight. It can be attached to the camera directly or use off camera to add additional light to a dark environment.
A flash is a device used in photography producing a flash of Artificial Light to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light. Flash refers either to the flash of light itself or to the electronic flash unit discharging the light. Most current flash units are electronic, having evolved from single-use flashbulbs and flammable powders. Flash units are commonly built directly into a camera. Some cameras allow separate flash units to be mounted via a standardized "accessory mount" bracket (Hot Shoe). In professional studio equipment, flashes may be large, standalone units, or studio strobes, powered by special battery packs or connected to power outlet. They are either synchronized (typically 1/200 of a second) with the camera using a flash synchronization cable or radio signal, or are light-triggered, meaning that only one flash unit needs to be synchronized with the camera, and in turn triggers the other units, referred to as slaves.
As photographers we’re always looking for perfect light. And yet, the quality of available light isn’t always ideal. It is rarely perfect. But in using flash wisely, photographer's are able to enhance or over-ride the available light. With careful use of flash, you are more in control of light, and hence the way the photos will look – than if you had just accepted the existing ambient light. Instead of waiting for perfect light, use what you have available, and add flash to make the best of the situation.