Lengthwise Versus Sideways:

Whenever waves travel via matter, the molecules oscillate to and fro, back and forth or up and down. The nature of the particle movement varies from the nature of the wave as it travels. The atoms or molecules seldom move more than a few meters-at times less than a centimeter-though the wave can travel thousands of kilometers. At times the particles vibrate in line with the direction of wave travel; this is a compression wave, also termed as a longitudinal wave. In another instance, the particles move at right angles to the direction of propagation; this is a transverse wave. The distinction is stated in figure shown below.

Figure: In a longitudinal wave (a), the particles vibrate parallel to the direction of wave travel. In a transverse wave (b), the particles vibrate laterally.

What is it that wiggles or wags or compresses or stretches whenever a wave travels via a specific medium? It depends on the medium and on the nature of the wave disturbance. The sound waves in air are longitudinal, though radio waves are transverse. The waves on the surface of the sea are transverse, though whenever a big wave arrives on a beach, abundance of longitudinal motion is included as well.