In the study of waves, there's many different types of waves. First, there are mechanical waves, such as sound, water, and seismic waves, which all require a medium to travel through. In contrast, there are electromagnetic waves, such as visible light and x-rays, which all don't require a medium. Also, when looking at waves, the crest is the top, such as the top of a wave on a beach, and the bottom is called a trough. The length between two crests is known as a wavelength and the amplitude is the distance from a medium to the crest, or to the trough. Also, waves have a frequency, which the number of waves in one second, and the period, which is the time of one wave.