Theory:

A system of units are the complete set of units, both fundamental and derived, for all kinds of physical quantities.
The common systems of units are given below:
  
System
Length
Mass
Time
CGS
Centimetre
Gram
Second
MKS
Metre
Kilogram
Second
FPS
Foot
Pound
Second
 
Important!
In the system of units, the first three systems of units (CGS, SI, MKS systems) are the metric systems of units. FPS is not a metric system of units: it is a British system of units.
SI Units:
The SI unit system is also known as the international system of units, and it is abbreviated from the French word 'systeme International'.
At the end of the II World War, there was a need to use a worldwide system of measurement. Hence, the SI (International System of Units) system of units was developed and recommended by General Conference on Weights and Measures at Paris in 1960 for international usage.
 
SI system of units are the modernised and improved form of the previous system of units.
 
It is accepted in almost all the countries of the world. It is based on a certain set of fundamental units from which derived units are obtained by multiplication or division.

There are seven fundamental units in the SI system of units, which are given below:
  
S.No
Fundamental quantities
Unit
Symbol
1
Length
Metre
m
2
Mass
Kilogram
kg
3
Time
Seconds
s
4
Temperature
Kelvin
k
5
Luminous intensity
Candela
cd
6
Electric current
Ampere
A
7
Amount of substance
Mole
mol
 
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SI Base units