Processes that took place through Earth's history, such as the weathering of rocks, evaporation of ocean water, and the formation of sea ice, have made the ocean salty. Those are still at work today and are counterbalanced by processes that decrease the salt in the ocean, like freshwater input from rivers, precipitation, and the melting of ice. The result is an ocean surface where the salinity - the concentration of salt - changes and these changes, small as they may be, have large-scale effects on Earth's water cycle and ocean circulation.
Looking down on the oceans from high above, this animation depicts the day-by-day time evolution of sea surface salinities simulated by the NOAA GFDL CM2.6 climate model. One can see the interplay of fresh waters as they seek to exit the Arctic, moving southward along both sides of Greenland until they merge and circulate around the rim of the Labrador Sea. At the same time, higher salinity surface waters flow from the Gulf Stream extension, pass between Iceland and the British Isles, and enter the Nordic Seas. Icebergs are made of fresh frozen water, therefore, where icebergs form, sea surface salinity increases causing the water to increase in density. This high resolution model - the model grid cells are all smaller than 5 km (3.1 miles) on a side in the Arctic region - produces a wealth of eddies, which play a role in transport salt and heat in the ocean.
Ocean salinity is measured in PSU (practical salinity unit), grams of salt per 1000 grams of water.