Gravity is one of the major causes of tides. The gravity of the moon and the sun pulls on the ocean's waters. Distance and mass are two factors of gravity. The greater the mass of an objects, the greater the gravitational pull on them. The closer two objects are, the greater the gravitational attraction between them. Although the sun is significantly larger than the moon, its contribution to affecting Earth's tides is significantly less that that of the moon's because the sun is so far away. Thus, the side of Earth that faces the moon (as well as the side of Earth that is opposite to the moon) experience a high tide because this side is closer to the moon. Likewise, the sides of Earth that are perpendicular to the moon will experience low tides.
A second contributing factor to Earth's tides is the centrifugal force produced as Earth orbits in space. Since the center of gravity is in the center, the centrifugal force is pulled "away from the center of revolution".