Tides are the periodic rises and falls of large bodies of water on earth. These changes in water depth are caused by the gravitational pull of the moon and the sun, along with the depth of the water, and barometric pressure. The most important force on the tides is the moon. The gravitational force of the earth attracts all the water in the oceans, but it is also attracted to the moon. The water's attraction to the moon is much smaller than that to the earth because the moon is much farther away and much smaller then the earth. The earth is also attracted to the moon but since the earth's surface is solid it is not easily distorted, and not as dramatically effected as the water. The water, being easily distorted by the moons gravity makes the tides. The attraction of the moon causes two bulges of water on opposite sides of the earth. The water on the side of earth closest to the moon has a large gravitational force on it and is pulled towards the moon, creating a large bulge on that side of earth. The bulge on the opposite side of earth forms because, although not as much as the water closest to the moon, the earth is being pulled towards the moon and away from the water on the far side, causing a bulge on the side of earth farthest from the moon also. Because our Earth rotates under these bulges, any given point on the surface will have two high tides and two low tides per rotation (approximately).
The sun also effects the earth's tides. Particularly high tides happen when the sun and the moon are lined up with the earth at new and full stages of the moon. These tides are called spring tides. The change in these tides is relatively the same whether the sun and moon are lined up on opposite sides of the earth or on the same side. The gravitational forces of the sun and moon acting together make a larger tide than average. Another type of tide that would not be possible without the sun is the neap tide.