The most important driving force behind tides is the gravitational pull of the moon. In fact, the moon is twice as important to the size of tides than the sun. Besides the sun and moon, the shape of the coastline, depth of the water and features on the ocean floor all affect tides.

We have different terms to describe different kinds of tides.Semi-diurnal tides occur twice a day. This means a body of water with semi-diurnal tides, like the Atlantic Ocean, will have two high tides and two low tides in one day. Diurnal tides occur once a day. A body of water with diurnal tides, like the Gulf of Mexico, has only one high tide and one low tide in a 24-hour period. Some bodies of water, like parts of the Pacific Basin, have mixed tides, where a single low tide follows two high tides.

Sometimes we have a very large difference between high and low tides. This phenomenon is called spring tides. For a spring tide to occur, Earth, the moon and the sun must all be in a straight line.
Very little differences between high and low tides are called neap tides. Neap tides happen when Earth, the moon and the sun form a right angle. The picture shows examples of how the three heavenly bodies must line up for neap tides to occur.

By measuring the difference between the high and low tides, we can find the tidal range. Tidal ranges can be measured in inches, like those in Lake Superior, Michigan., or tidal ranges can be measured in feet, like those in the Bay of Fundy. In fact, the Bay of Fundy, a Canadian inlet on Nova Scotia's West Coast in the Atlantic Ocean, has the greatest tidal range known, up to 50 feet!

Associated with tides is a current that runs horizontally. This tidal current can be described as either ebb or flood. Tidal currents moving away from the beach are ebb currents. Tidal currents moving toward the beach are flood currents.

Information provided by: http://www.onr.navy.mil