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.
Now suppose the sun and the moon were both on the same side of the earth, like during a new moon. That would mean there's even more of a gravitational pull in that direction than usual. This causes especially high high tides--called spring tides. The same thing happens during a full moon, when the sun and moon line up on opposite sides of the planet--each pulling from both ends.
When the moon is in a quarter phase, the sun and moon are at a ninety degree angle to each other. During this phase, the gravitational pulls are canelled out, producing a smaller difference between high and low tide--also known as a neap tide. Spring tides and neap tide levels are about 20% higher or lower than average.
And because the tides are brought on by waves with a very long wavelength (the distance between the "crest" or tips of waves), they are affected by their interaction with the seafloor. Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet. But in shallower water, these waves collapse upon themselves as they come in contact with the sea floor. The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified. (In the Bay of Fundy, tides have a range of 44.6 ft.)
In most places on the planet, high and low tides occur twice daily. Each day these tides change 50 minutes later, as it takes the moon 24 hours and 50 minutes to completely rotate around the earth.