High tide is basiclly when the moon is most over head. If we were to have a world with no land interfereing with the flow of tides, directly under the moon, the water would bulge towards it's pull. There is one high tide. At the excat oopposite of the earth there is another high tide there. The reason for this is what we call centrifugal force.
Centrifugal force tends to propel things outward.
There are normally two high tides during the day. So we said, the sun is a junior partner in the flow of the ocean, it's power is less than half the moon's, and all it basiclly does is increase or decrease the moon's power over the oceans. Everyday the tide rises fifty five minutes later than the day before. This has to do with the moon's revolution around the earth. The above was the fundemantals of tides, for more detailed information see the links.