Salinity is a general term used to define the degree of saltiness of a particular body of water or mass of soil. In ocean, salinity is usually termed as the concentration of sodium chloride (and/or bicarbonates, magnesium and calcium salts). 

There are differences in the salinities in the bodies of water around the world mainly because of the water cycle. 

• Rate of evaporation - the more increased the rate of water vaporization, the more saline the water will be. As the evaporation continues, the surface will be more concentrated.
• Rate water flow - there are bodies of water that are somehow more isolated than the other. If the body of water continues to flow, dissolved salt particles can be transferred to other places thus diluting the original source. But when the flow of water is not efficient, the salt particles will build up thus increasing the salinity. 
• Isolated water - For example, the great salt lake and the red sea have salinity which is approximately 10 times with the salinity of seas. This is because the water is isolated and there are no points of transfer other than evaporation. When you evaporate the salt lake, only water is removed, the salt remains.