Ocean currents are continuously flowing water that can occur on the surface of the ocean, or very deep under the surface. Surface currents are usually caused and controlled by the wind, while deep water currents may be caused by things such as temperature, salinity, density, and the rise and fall of the tides, which in turn is caused by the gravitational pull of the Sun and the Moon. Since surface currents are caused by the winds, they are less predictable than the deep water currents. Deep ocean currents, however, are more predictable, and have global impacts. These currents are controlled by density, which is turn affected by the temperature, depth, and saline level of the water. The water of the deep currents is made denser as it gets colder and saltier. This causes a “mixing” of the surface and deep ocean currents, causing a global-scale circulation system that is known as the global conveyor belt. This global circulation affects the climate by carrying warmer waters and then heating the environment along the way. The Gulf Stream is one example, as the water is moved from the equator up to the coast of Western Europe, making those areas warmer than some equal northern latitude countries.
Sources:
http://education.nationalgeographic.com/education/media/ocean-currents-and-climate/?ar_a=1
http://education.nationalgeographic.com/education/encyclopedia/ocean-conveyor-belt/?ar_a=1
http://en.wikipedia.org/wiki/Ocean_current
http://oceanservice.noaa.gov/education/tutorial_currents/welcome.html
http://www.sciencedaily.com/articles/o/ocean_current.htm
http://www.ducksters.com/science/earth_science/ocean_waves_and_currents.php