1. The general movement of a mass of water in a fairly defined direction over great distance is called ocean current.

2.Causes that lead to the formation of ocean currents are : 

a) Centrifuge! Force : 

The centrifugal force at the equator is greater than that at the poles. The variation of these forces makes the equatorial water to move towards the poles. 

b) Effect of Winds : 

The stresses due to wind and the wind movement modifies the the theoretical direction of currents. Due to the frictional gliding of winds water is dragged along the wind direction. Thus a 50 miles per hour wind will produce a current whose velocity is 0.75 miles per hour.

c) Effect of Precipitation :

Since the level of sea is higher water moves north and south from the equator. 

d) Effect of Solar Energy :

Heating by solar energy causes the water to expand near the equator. Hence near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope. 

e) Salinity, density differences, melting of ice also affect the ocean currents