Tides are created by the gravitational pull of the moon and, to a lesser extent the sun, upon the Earth's oceans. The sun has about half the effect that the moon has on the Earth's tides.

In most areas, typically two high tides and two low tides occur within a 24 hour and 50-minute lunar day. (A lunar day is 50 minutes longer than a solar day.) The time between each high and low tide (or low and high tide) is 6 hours and 12.5 minutes.

In most coastal regions, one of the daily high tides is higher than the other high tide and one of the low tides is lower than the other.

Some regions, such as the Gulf of Mexico, only have one high tide and one low tide each day.

During a new or full moon, both the sun and moon are in line with the Earth. The combined gravitational effect causes extremely high-high and low-low tides. The tides during this time are referred to as spring tides.

The difference between the highest and lowest tides is called the tidal range. When people speak of the tide as being "in" or "out," they are referring to the cyclic changes of the water level within this tidal range. The intertidal zone is the shore area within the tidal range. This zone gets exposed to a wide extreme of conditions. Although the intertidal zone is a relatively limited area, a wide variety of plants and animals make their homes here.

Scientists have divided the intertidal area (also called the littoral zone) into four main zones: