The tide is the regular rising and falling of the ocean's surface caused by the Moon's gravity (and to a lesser degree the Sun's).
The maximum water level is called high tide; the minimum level is low tide. At any given point on the ocean, there are normally two high tides and two low tides each day. On average, high tides occur 12 hours 24 minutes apart. The 12 hours is due to the Earth's rotation, and the 24 minutes to the Moon's orbit.
The height of the high and low tides (relative to mean sea level) also varies. Around new and full Moon, the tidal forces due to the Sun reinforce those of the moon. The tide's range is at its maximum: this is called the spring tide, or just springs. When the Moon is at first quarter or third quarter, the forces due to the Sun partially cancel out those of the Moon. At these points in the Lunar cycle, the tide's range is at its minimum: this is called the neap tide, or neaps.
In most places there is a delay between the quarters of the moon and its effects on the tide on geographical locations around the globe. Springs and neaps in the North Sea for example are 2 days behind respectively new/full moon and first/third quarter. Reason for this is that the nature of the tide originates in the Southern Ocean because this is the only place on the globe where a circumventing wave (as caused by the tidal force of the moon) can travel unprohibited by land.
The resulting effect on the amplitude or height of the tide travels across the oceans. It is known that it travels as a standing wave northwards over the Atlantic. This causes relatively low tidal differences in some locations (knots) and high ones in other places. This is not to be confused with local geography as can be found in Nova Scotia, Bristol, the Channel Islands and the French Channel (La Mange) coast. In these places tidal differences can be over 10 meters.
The Atlantic tidal wave arrives after approximately a day in the Channel area of the European coast and needs another day to go around the British islands in order to be effective in the North Sea. Peaks and lows of the Channel wave and North Sea wave meet in Dover Strait / Pas de Calais at about the same time but generally favour a current in the direction of the North Sea.
The exact time and height of the tide at a particular coastal point is also greatly influenced by the local topography. There are some extreme cases: the Bay of Fundy, on the east coast of Canada, features the largest tidal range in the world, 53 feet (16 meters), because of the shape of the bay. Southampton in the United Kingdom has a double high tide caused by the flow of water around the Isle of Wight, and Portland has a double low tide. Also there is little tide in the Mediterranean due to the narrow connection with the ocean.
It is often assumed that the tides are simply the Moon's gravitational force pulling the oceans' water toward itself, but this is wrong. Were it so, there would only be one high tide every 24 hours (imagine the water around the Earth with a single bulge pointing towards the Moon). Instead, the tide is caused by tidal forces, which are due to the difference in gravitational attraction on the near and far sides of a body. The tidal force produces two bulges: one pointing towards the Moon, and one pointing away. This is also why the Moon is the major cause of the tides: at the Earth's surface the straightforward gravitational attraction of the Sun is considerably larger than that of the Moon, but the difference in the Moon's grativational force from the near side of the Earth to the far side is much greater than the Sun's.
The tidal forces fall off according to an inverse cubic law: the gravitational forces themselves are proportional to the inverse square of distance, and the significance of a difference in distance falls inversely with distance. The much greater distance of the Sun makes its tidal forces on the Earth much smaller than the Moon's.
Tides also affect the shape of the Earth itself, not just its oceans. These "land tides" are not as pronounced as the ocean tides, however, due to the reduced flexibility of Earth's crust and mantle. Land tides are also delayed about two hours relative to ocean tides due to this stiffness.
The first mathematical explanation of tidal forces was given in 1687 by Isaac Newton in the Philosophiae Naturalis Principia Mathematica.
Tsunami, the large waves that occur after earthquakes, are often called tidal waves, but have nothing to do with the tides.
See also: coastal erosion, storm tide