Most of the time, there are two tides each day. The understanding of this complex phenomenon is not simple and many great scientists (Aristotle, Galileo, Newton, Lagrange) attempted to attain that understanding. Gravitation is the force responsible for this phenomenon. The relative positions of the Moon, the Sun and the Earth explain the observed variations in this phenomenon.
Finally, the form of the littoral plays an important role in explaining the different amplitudes observed on the coasts.