World's Largest Tidal Bore, Qiantang River, China

Tides are the rise and fall sea levels caused by the combined effects the gravitational forces exerted by the Moon and the Sun and the rotation the Earth. Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude the tides at the coast are influenced by the alignment the Sun and Moon, by the pattern tides in the deep ocean and by the shape the coastline and near-shore bathymetry.
Most coastal areas experience two high and two low tides per day. The gravitational effect the Moon on the surface the Earth is the same when it is directly overhead as when it is directly underfoot. The Moon orbits the Earth in the same direction the Earth rotates on its axis, so it takes slightly more than a day—about 24 hours and 50 minutes—for the Moon to return to the same location in the sky. During this time, it has passed overhead once and underfoot once, so in many places the period strongest tidal forcing is 12 hours and 25 minutes. The high tides do not necessarily occur when the Moon is overhead or underfoot, but the period the forcing still determines the time between high tides.
The Sun also exerts on the Earth a gravitational attraction which results in a (less powerful) secondary tidal effect. When the Earth, Moon and Sun are approximately aligned, these two tidal effects reinforce one another (resulting in higher highs and lower lows). This alignment occurs approximately twice a month (at the full moon and new moon). These recurring extreme tides are termed spring tides. Tides with the smallest range are termed neap tides (occurring around the first and last quarter moons).
Tides vary on timescales ranging from hours to years due to numerous influences. To make accurate records, tide gauges at fixed stations measure the water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level.