Flash in digital photography is a form of extremely bright light used to illuminate a scene, or to help freeze fast moving objects. Flash refers to both the flash of light itself and to the electronic flash unit actually producing the light.
Flash light has a color temperature of around 5500k (kelvins), and a burst of light usually lasts between 1/1000th and 1/200th of a second.
Modern flash light is electronic, having evolved from single use flashbulbs and, originally, from flammable powder! There are several different kinds of flash. Compact digital cameras have a built-in flash. DSLRs sometimes have a built-in "pop-up" flash. Typically, the pop-up flash is not very powerful. All DSLRs allow for the attachment of an external flashgun -- via the hotshoe (a standard accessory bracket located on the top of the camera).
Professional photographers also use stand-alone flash units, known as studio lights, which either connect to power outlets or are powered by battery packs.
Electronic flashes have a tube which is filled with xenon gas. High voltage electricity is discharged, which generates an electrical arc. The arc then emits a short flash of light. The strength of a particular flashgun is measured by a guide number.