In longitudinal waves, the vibrations are parallel to the direction of wave travel.
Examples of longitudinal waves include:
One way to remember the movement of particles in longitudinal waves is to say the waves move along the same way as the particles.
Longitudinal waves are often demonstrated by pushing and pulling a stretched slinky spring.
In the diagram, the compressions move from left to right and energy is transferred from left to right. However, none of the particles are transported along a longitudinal wave. Instead, they move backwards and forwards between compressions as the wave is transmitted through the medium.
Sound waves are longitudinal waves. They cause particles to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases, by a series of compressions and rarefactions.
Sound cannot travel through a vacuum because there are no particles to carry the vibrations.