A wave is a form of motion. Often waves move forms of energy, such as light, from place to place. The molecules in a substance vibrate as the waves move along. Many things travel in waves, including water, sound, light, television and radio waves, and tiny particles of radiation. A wave is called transverse if the particles in the wave move up and down. Water and light waves move in this manner. A wave is longitudinal if the particles vibrate in the same direction as the movement of the wave. Sound waves move in this way. The length of a wave is the distance between each high or low point on the wave's curve.