Tides are caused by the proximity of the
moon to the earth. They are a result of the
gravitational pull between the two bodies; water, being easier to move than land, is more obviously affected.
There are two high-tides each day; the side of the earth nearest the moon has a direct high-tide, while the opposite side of the earth has an indirect high-tide. Low tide occurs 90 degrees away from either of those. The two high-tides occur approximately 12.5 hours apart, and are of equal duration. These are called semi-diurnal tides. Of course, in some weird parts of the world, the structure of the sea basin, coastlines, proximity to the equator, or other geographic features, one tide is noticeably higher or longer than the other. In some cases, there is only one high-tide each day; this is called a diurnal tide and has a frequency of 24 hours, 50 minutes.
There is a range of tides which represent the differences in height of high tide and low tide. Spring tide has the largest range (greatest difference) and occurs during the full moon. This is a result of the sun, earth, and moon being in alignment, a condition known as syzygy. The lowest range (least difference) is called neep tide and occurs during the first and last quarters of the moon. Neep tide is low because the sun, moon, and earth form a right angle, and the gravitational effects of these celestial bodies interfere with one another.