The tides--the daily rise and fall of the sea's edge--are caused by the gravitational forces between the earth, the moon and the sun. Since the moon is closer to our planet than the sun, it exerts a stronger gravitational pull on us. (The sun only has 46% of the tide-generating force of the moon.) So when the moon faces one side of the earth, it pulls on all the earth's surfaces, but since only the ocean is flexible, only the ocean succumbs to its force. This forms a bulge on the side of the planet facing the moon, while the centrifugal force from the earth's rotation causes a bulge to form on the other side. Where these bulges occur, it's high tide.

http://www.umassd.edu/Public/People/Kamaral/thesis/Tides.html

The Tides