Ocean tides are the reptitive rise and fall of sea levels relative to the land. In any given day, there are two low and two high tides, with the water constantly moving between the two extremes. Each cycle from high to low to high takes around 12 hours and 25 minutes. The times of high and low tides change by around 50 minutes from one day to the next, as the moon takes more than 24 hours to complete one circuit of the sky.
Tides occur due to the gravitational attraction between the Earth and the moon. The two bodies are constantly pulling at each other, and the gravitational attraction causes a bulge in the ocean on the side of the Earth closest to the moon. Since the Earth is also being pulled toward the moon (but less so than the water) another bulge of water appears on the opposite side, where the Earth is being drawn away from it. These bulges form high tides, with low tides occuring in between. As the moon orbits around the Earth and the Earth itself rotates, the high tides move to new locations around the world. Although the effect of the moon is weak - about one ten millionth of the gravitational force of the Earth - other factors, such as the centrifugal force generated by the Earth's rotation, add to it such that its pull on the oceans is slightly greater than that of the Earth.
The sun can combine with the moon to affect the tides, though its gravitational effect on the Earth is less than half that of the moon. When they are aligned (when the moon is new or full), their combined gravitational force causes very high and very low - or spring - tides. When the sun and moon are at right angles with respect to the Earth (during the moon's quarter phases), their gravitational forces cancel each other out and the tidal movements are lessened to form neap tides. Spring and neap tides are generally 1/5 higher or lower than normal.
There is another type of tide called a proxigean spring tide, which is a rare, extreme form of spring tide. It occurs when the moon is new (between the Earth and sun) and at its proxigee. The proxigee is the point of the moon's elliptical orbit that is closest to the Earth, and is 92.7% of its average distance. This produces a 25% increase in the tide. The proxigean spring tide happens once every 1.5 years or more.