A wave is a disturbance that travels through a medium from one location to another, transferring energy but not matter. There are two main types of waves - transverse waves, where the disturbance moves perpendicular to the direction of energy transfer, and longitudinal waves, where the disturbance moves parallel. Sound is an example of a longitudinal wave, as it requires a medium like air or water to travel through and compresses and expands the particles of the medium as it moves.