The earliest flashes consisted a lump of magnesium powder that was ignited by hand. Later, the magnesium were contained in flash bulbs, and were electrically ignited by a contact in the camera shutter. A bulb could only be used once.
For the Kodak Instamatic camera, a flash cube of 4 bulbs were introduced, that allowed taking 4 images in a row. The later X-cube was noteworthy in that it also contained a piezo-electric element that generated the electricity required to fire the bulbs, so that a battery was not required.
Todays flash units are fully electronic, and contains a Xenon gas filled tube where electricity of high voltage is discharged to generate a short flash of light. The typical duration of the light impulse is 1/1000 second. The majority of modern cameras now have a flash unit built in.
A separate flash unit may usually be mounted to a camera via a standardised accessory mount bracket.