The term "current" describes the motion of the ocean

The Kuroshio Current, in the Pacific off the east coast of Taiwan extending northward off the east coast of Japan, is the ocean's largest current. It can travel between 40-121 kilometers per day (25-75 miles per day) at speeds between 1.6-4.8 kilometers per hour (about 1-3 miles per hour) and extends some 1,006 meters ( 3,300 feet) deep.

Oceanic currents are driven by three main factors: tides, wind, and temperature changes.

1. The rise and fall of the tides. Tides create a current in the oceans, near the shore, and in bays and estuaries along the coast. These are called "tidal currents." Tidal currents are the only type of currents that change in a very regular pattern and can be predicted for future dates.

2. Wind. Winds drive currents that are at or near the ocean's surface. These currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour). Winds drive currents near coastal areas on a localized scale and in the open ocean on a global scale.

3. Thermohaline circulation. This is a process driven by density differences in water due to temperature (thermo) and salinity (haline) variations in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents.

Currents affect the Earth's climate by driving warm water from the Equator and cold water from the poles around the Earth. The warm Gulf Stream, for instance, brings milder winter weather to Bergen, Norway, than to New York, much further south. It keeps the Norwegian coast an incredible 6.1 degrees Celsius (43 degrees Fahrenheit) warmer than other places equally far north.

Most ocean currents flow in one direction all the time. In the northern Indian Ocean, though, they change direction twice a year, driven by the monsoon winds. From November to March, the currents are blown towards Africa by the cool, dry north-east monsoon winds. In May, the winds blow in the opposite direction, driving the water towards India.

For more information:
Observations of the Gulf Stream, NOAA Ocean Explorer
NOAA CoastWatch
Tides & Currents: Center for Operational Oceanographic Products and Services