How Tides Work

The universal law of gravity states that the attraction between two bodies is proportional to their respective masses and inversely proportional to the distance between them. By definition, "The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted by one body on another is not constant across it; the nearest side is attracted more strongly than the farthest side".  Gravity is the primary force involved in creating the "tidal bulge" on the earth's surface. The ocean is attracted to the moon, pulling water away from one area, and to another. This effect is most drastic on the side of the earth closest to the moon. As the earth rotates, the solid earth spins as if suspended in the ocean itself. This daily rotation is responsible for the twice a day high and low tides. If the earth were free of land masses, the ocean as a whole would move in perfect harmony with the gravitational pull of the moon. Tides would be equally proportional with respect to individual lunar days on the global scale. The manifestation of this force on the oceans is affected on multiple orders of magnitude. On the largest scale, continental bathymetry (sub-marine shape of continents) has the largest impact on tides in the regional perspective (Pacific northwest vs south Atlantic). As we look closer, the width and depth of individual bays is a scalar for the tidal forces in the regional area. A classic example of this is cook inlet here in Alaska (background image). Photos: The first one is from google earth, the second one is from the youtube video "understanding tides" http://www.youtube.com/watch?v=G-MOZId0FNw , and the third one is from wikipedia