An incandescent bulb or incandescent light bulb is the "classic" light bulb. It works by passing electricity through an electrical filament which then releases the energy in the forms of light and heat.
Most incandescent bulbs give off about 90% of the energy used in heat rather than visible light. So incandescent bulbs are being phased out in favor of compact fluorescent bulbs in many places in favor of more efficient lighting such as LEDs.
- Wikipedia's article on incandescent light bulb