A current is a steady flow of a water or air inside a larger body of water or air. Prevailing winds are currents of air that blow in mainly one direction. The pattern of prevailing winds is caused by the uneven heating of the earth.
When prevailing winds blow over the surfaces of bodies of water, they create surface currents. The air currents above the oceans and the surface currents of water look curved because of the rotation of the earth. This is called the Coriolis Effect.
Surface currents connect and form large spinning water systems known as gyres that circulate heat around the world by taking warm water from the equator to the poles.
The oceans also have deep currents that are influenced by differences in density of the water. Colder and saltier water that is denser sinks into the ocean. Warmer and less salty water rises because it has less density.
These vertical currents are connected by horizontal currents at the surface and deep in the ocean. This system of currents is called the overturning circulation.