Tides

Tides are a natural phenomena that occur all over the world. They are known as the rise and fall of the sea around the coasts of land. Tides are caused by the gravitational pull/attraction that the moon and sun have with the earth. There are two types of tides, spring tides and neap tides. Spring tides are those tides that are very high and very low and occur twice a month. They occur with the new moon and the full moon. Neap tides are those that are halfway inbetween the phases of the moon.
Tides that occur far out in the middle of the sea go unnoticed, but those that occur around the shores and beaches are very noticed and and govern many water related activities, commercial and recreational. Depending on the place will depend on ow the tides behave. In places where there is one high tide and one low tide a day is said to be diurnal tides. In these areas, the maximum water level is called high tide and the minimum water level is low tide. Semidiurnal tides there is a cycle of two high water low water sequences. In this case, the two high tides reach about the same height and the two low tides drop to about the same level. In areas where the high tides reach different hieghts and the low tides drop to different levels is sadi to have semidiurnal mixed tides. In these areas, the highest maximum water level is called higher high tide, the lowest maximum water level is called lower high tide, the highest minimum water level is called higher low tide and the lowest minimum water level is called lower low tide.

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~ Chapter 11 in 'An Introduction to the World's Oceans'.
~ http://en.wikipedia.org/wiki/Tide
~http://www.physicalgeography.net/fundamentals/8r.html