Question

What device on digital cameras are the size of postage stamps, come in different speed and capacity levels, and are capable of storing a great deal of data?

Answer 1

Those are removable flash memory cards — commonly SD cards (Secure Digital). They're about the size of a postage stamp, come in different speed classes and capacities (SD, SDHC, SDXC, UHS classes, etc.), and store your camera photos and video.