What creates tides in the oceans?

As MacG correctly put it, tides on Earth's oceans are caused by the pull of gravity from both the moon and the sun. When there is a new or full moon as seen from Earth's surface, the tides are at their highest; this is because the gravitational forces of the sun and moon are in line with each other and so add up. When the moon looks only half illuminated from Earth's surface, the moon and the sun form a right angle in space with Earth at the corner. At these times the gravitational pulls of sun and moon partially cancel each other out (as they point to different sides of the Earth), and so the tides are at their lowest.