Before the advent of the modern camera flash with its Xenon flashtube, flashbulbs were used to provide illumination for photography.
Flashbulbs replaced the more dangerous and finicky flash powder which had been in use since the dawn of photography.
Flashbulbs come in many sizes, from large sizes for illuminating large, outdoor scenes to tiny sizes for close range interior work.
Flashbulbs should be used with a protective shield, as defective bulbs are prone to explode.
Though used in much lower quantities now, they are still being produced by small factories (like Meggaflash). Silvania, the last major manufacturer, has stopped production. If you buy flashbulbs now, it is very likely you are buying what is left in warehouses.
Flashbulbs are still one of the most compact ways of generating light. Because of this, certain kinds of photography still require them. I've seen pictures of underground caves lit by a single high-power flashbulb, which would have been nearly impossible with normal electric flashes.