The bottom of a wave, or a low spot between waves, is referred to as the trough, while the top of a wave is referred to as the crest. As waves elevate, the force of gravity pulls them back down, causing the crest of a new wave to form.
Most waves are caused by steady winds or in storm centers. In a storm's center, waves of different sizes and lengths occur at once. As the waves travel outward from the center of the storm, they become more predictable because the longer wavelengths travel at a faster rate than the shorter wavelengths. Long wavelengths that are spaced out and predictable are referred to as swells.