Waves are a transfer of energy. Electromagnetic waves do not need a medium to travel through but the three types of mechanical waves do. A medium is a substance the waves move through.
3 Types of Waves
Transverse waves move up and down. The crest is the highest points at the top of the wave above the equilibrium, while the trough is the lowest points of the wave under the equilibrium. The amplitude either extends from the crest to the equilibrium or from the trough to the equilibrium, but never from crest to trough. The wavelength is from one crest to the next crest or from one trough to the next trough.
Longitudinal waves move side to side. Rarefaction occurs with low pressure or when the waves seem to be spread further apart. Compression occurs with high pressure when the waves appear to be closer together. The wavelength of a longitudinal wave is from one compression spot to the next compression spot or from one rarefaction spot to the next rarefaction spot.
Two media are in contact, air and water. Surface waves move in a circular motion coming out of the waves and returning back in. Also have an up and down movement like transverse waves.