Transverse waves have two parts: a crest, which is the highest point of the wave, and the trough, which is the lowest part of the wave. Longitudinal waves also have two parts: compression, which are areas of high molecular density, and rarefactions, which are areas of low molecular density.
There are two types of waves depending on the direction of motion of the wave relative to the direction in which the particles vibrate. In transverse waves, the direction of the wave energy is perpendicular to the direction of the movement of the particles. In other words, if the wave is traveling from left to right, the particles move up and down. The highest point a particle reaches in a transverse wave is called a crest while the lowest point is called a trough. In longitudinal waves, the direction of the wave energy is parallel (or in the same direction) to the movement of the particles. In other words, the particles vibrate back and forth along the direction of the wave itself. The areas of the wave where the particles are close together are called compressions. This means that there must be areas of the wave where the particles are spread further apart from each other. These areas are called rarefactions. For a transverse wave, a complete wave cycle, or wavelength, must include one crest and one trough. For a longitudinal wave, a complete wavelength must include one compression and one rarefaction.