The direction of these oscillations is the difference between longitudinal and transverse waves. In longitudinal waves, the vibrations are parallel to the direction of wave travel. In transverse waves, the vibrations are at right angles to the direction of wave travel.
Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through. Electromagnetic waves cause oscillations in electrical and magnetic fields.
It is important to remember that all waves transfer energy but they do not transfer matter. For example, if a ball is placed on the surface of a pond when ripples move across it, the ball will move up and down but not outwards with the wave. Similarly, when sound waves move through the air to a person's ear, the air itself does not move - instead the sound is transferred through the vibrating molecules, which stay where they are and vibrate about a fixed point.
Waves are described using the following terms: