Sounds are longitudinal vibrations travelling in solids, liquids or gases.
Sound cannot travel in a vacuum because there is nothing for the vibrations to pass through.
Sound travels fastest through things that have tightly packed atoms: solids.
The speed of sound is slower in liquids than solids, and slowest in gases.
The speed of sound in air is around 340 m/s, in water about 1500 m/s and in solids around 5000 m/s.
Lightning causes thunder. The delay between them happens because the speed of light is nearly 1,000,000× that of sound in air.