Organisms that can create their own light are bioluminescent. Though they're endangered, there are still many places in the world you can see bioluminescent fireflies blinking brightly in the dark.
A firefly or lightning bug's illumination is generated in its abdomen, where a chemical reaction produces photons that transmit light. The process varies among the many marine animals, fungi, and bacteria that are bioluminescent. A glowing squid native to the Pacific Ocean gets help from bioluminescent bacteria living inside its body, for example. Bio- means "relating to a living thing," and luminescent derives from the Latin lumen, "light."