Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it.
However, most of the electrical energy is transferred as heat energy instead of light energy.
Modern energy-saving lamps and LEDs (light-emitting diodes) work in a different way.
They transfer a greater proportion of electrical energy as light energy.
This is the Sankey diagram for a typical LED lamp:
From the diagram, you can see that much less electrical energy is transferred, or 'wasted', as heat energy from the energy-saving lamp.
It's more efficient than the filament lamp.