SI UNITS:
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around 7 base units and the convenience of the number ten.
A quantity in the general sense is the property ascribed to phenomena, bodies, or substances which can be quantified for, or assigned to, a Particular phenomenon, body, or substance. Examples are electric charge and mass.
A quantity in the particular sense is a quantifiable property ascribed to a particular phenomenon, body, or substance.
Examples are the mass of moon and electric charge of the proton.
A physical quantity is a quantity which can be used in the mathematical equations of science and technology.
A unit is a particular physical quantity, which is defined and adopted by convention, with which other particular quantities of same kind are compared to express their value.
The value of a physical quantity is quantitative expression of the particular physical quantity as the product of a number and a unit, the number being its numerical value. Thus, the numerical value of a particular physical quantity depends on the unit in which it can be expressed.