| Tides are caused by the gravitational pull
of the moon. As the moon revolves around the earth its gravity pulls
earth's oceans towards it, causing the water level to rise.
You may ask yourself why it is that we experience high tides twice a day (each 12 hours) even though the moon only circles the earth once a day. As gravity produces a water bulge under the moon, so too does it produce a bulge directly opposite the moon. This is due to the lessened effect of the earth's gravity on the water opposite the moon. In fact, the moon produces a differential force, pulling more forcefully on the earth (2) and the water closest to it (3) because they are closer, than on the water opposite it (1) which is farther away.
There are many other factors influencing the tides, two of the most noticable cause the spring and neap tides. Spring tides occur when the sun and the moon are lined up, either on the same side of the earth or on opposite sides. Neap tides occur when the moon is perpendicular to the sun. Neap tides are lower than either regular tides or spring tides because the sun's gravity cancels out some of the moon's pull.