Measuring is not simply counting. For example if we need milk or sugar we must also understand how much quantity of milk and sugar we are requared. Thus, thereis a need of som standard quantities for measuring comparing unknown quantities. Once a standard is set for a quantity then it can bi expressed in terms of that standerd quantity. This standard quantity is called a unit. 
With the development in the field of science and techonology, the need for a commonly acceptable system of units was seriously felt all over the world particularly to exchange scientific and technical information. The eleventh general conference on weight and measures held in Paris in 1960 adopted a world-wide system of measurements called international system of units. The intetnational system of units is commonly referred aa SL.
There are two types of units.
- Base units
- Derived units
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