Tides are caused by a combination of three factors: the moon's gravity, the sun's gravity and the earth's own gravity. The moon's gravity is the most significant of these forces; it exerts 2.2 times more force on the tides than the sun's gravity does.
Lunar and solar tides occur in regular cycles. Lunar tides occur over a 24 hour and 50 minute cycle, while solar tides occur over a 24 hour cycle. In addition to these daily high- and low-tide cycles, the monthly lunar cycle produces what are called spring tides and neap tides.
Spring tides occur at the new moon and the full moon. All three gravitation forces that cause the tides are aligned at spring tide, causing a greater than average difference between high and low tides. Neap tides occur at the half moon and three-quarters moon; at neap tide, the sun and moon are 90 degrees apart. Their gravitational forces work against each other, causing a smaller than average difference between high and low tides.
The world's smallest tidal distances occur in the Mediterranean Sea, where the difference between high and low tide is less than 3 feet. The greatest distances are found in the Bay of Fundy in Canada, where the difference can exceed 50 feet.