Question

What is the key difference between mechanical and electromagnetic waves?

Answer 1

Mechanical waves require a medium in order to move energy from one location to another. EM waves can operate in a vacuum.
The elastic and inertial properties of the medium determine the wave speed for mechanical waves.

Answer 2

Waves that does not need any medium or matter to travel through, and travels through space, are called electromagnetic waves. Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving.