The oceans cover the majority of the planet's surface. Water has a very high heat capacity, meaning that it can retain absorbed heat well without it quickly dissipating into the atmosphere. As a result, the oceans are a key driver of heat flows and transfers across the different parts of the planet. The equatorial regions naturally receive more direct sunlight throughout the year than the polar regions, and ocean currents (and wind currents as well) help to more evenly distribute this heat imbalance across the planet, especially in terms of keeping the mid-latitudes temperate.
Ocean currents can be divided into two major categories, surface and deep. Surface currents are mostly wind driven. Deep ocean currents are driven by salinity gradients, tidal influence of the Moon and Sun, and rotation of the planet (the Coriolis effect). In addition, density differences in cold versus warm water cause vertical rising and falling of water currents (updrafting and downdrafting) in the deep ocean.
The effects of ocean currents are numerous. They help mix ocean nutrients to feed animals. They also help migratory ocean animals to travel across the globe. They also affect weather patterns on the planet. The Gulf Stream is a prime example. The Gulf Stream is a deep ocean current that travels from the tropics up the North American east coast and then crosses the Atlantic toward northeastern Europe. This current has the effect of bringing relatively warm water to Ireland and the British Isles. The UK sits at the same latitude as Scandinavia and Russia yet experiences a less harsh winter then they do. This is in part due to the fact that the Gulf Stream keeps their coastal waters slightly warm than the surrounding areas.
Ocean currents are another reason why global climate change is an important topic. The melting of polar ice can alter salinity levels of ocean waters, thus effecting global deep ocean currents and the weather patterns they influence.