An international system of units, called SI units, was adopted at the 11th General Conference on Weights and Measures (CGPM) in 1960. SI is an abbreviation of the French name  Le Systeme Internationale de Unite’s.
Measurements are concerned with quantities like length, mass, time, density, etc. Any quantity which can be measured is called a physical quantity. The SI system of units is based on seven base units corresponding to seven base physical quantities. These are the physical quantities, in terms of which other physical quantities can be measured.
#  Physical Quantity  Symbol  SI Unit  SI Unit Symbol 
1  Length  l  metre  m 
2  Mass  m  kilogram  kg 
3  Time  t  second  s 
4  Electric Current  I  ampere  A 
5  Temperature  T  kelvin  K 
6  Amount of Substance  n  mole  mol 
7  Luminous Intensity  I  candela  cd 
The other measurements for temperature are in degree celsius (°C) and Fahrenheit (F).
SI Prefixes
When we make measurements of physical quantities, quite often the quantity being measured is too large as compared to the base unit of the physical quantity. For example,
Mass of earth = 5,970,000,000,000,000,000,000,000 kg
Radius of Sun = 6,96,000,000 m
Approximate distance between Mumbai and Delhi = 1,400,000 m
Other possibility is that the physical quantity is too small as compared to the base unit of the physical quantity. For example,
Radius of a hydrogen atom = 0.000,000,000,05 m
Mass of an electron (m_{e}) = 0.000,000,000,000,000,000,000,911 kg
When the physical quantity being measured is either too large or too small as compared to the standard unit, then the value of the physical quantity is quite inconvenient to express. The numbers can be simplified by using scientific notation of numbers. In this notation system we represent the numbers as power of ten.
Mass of Earth = 5.97 × 10^{24} kg
Radius of Sun = 6.96 × 10^{8} m
Approximate distance between Mumbai and Delhi = 1.4 × 10^{6} m
Radius of a hydrogen atom = 5 × 10^{11} m
Mass of an electron (m_{e}) = 9.11 × 10^{31} kg
In scientific notation the numbers become relatively easier, but are still not convenient because they carry exponents. In order to simplify the numbers further, the SI system of units has recommended the use of certain prefixes. These prefixes are used along with the SI units in such a way that the physical quantity being measured can be expressed as a convenient number. The SI prefixes have been defined to cover a wide range of 10^{24} to 10^{+24} of a unit.
Multiple  Prefix  Symbol 
10^{24}  yotta  Y 
10^{21}  zetta  Z 
10^{18}  exa  E 
10^{15}  peta  P 
10^{12}  tera  T 
10^{9}  giga  G 
10^{6}  mega  M 
10^{3}  kilo  k 
10^{2}  hecto  h 
10^{1}  deca  da 
10^{1}  deci  d 
10^{2}  centi  c 
10^{3}  milli  m 
10^{6}  micro  μ 
10^{9}  nano  n 
10^{12}  pico  p 
10^{15}  femto  f 
10^{18}  atto  a 
10^{21}  zepto  z 
10^{24}  yocto  y 
In order to use SI prefixes, you have to keep a basic rule in mind. The rule is that the prefix is chosen in such a way that the resulting value of the physical quantity has a value between 0.1 and 1000.
Rules for SI Prefixes

No space is required between the prefix and the symbol of the unit. For example, nanogram is written as ng and not as n g.

The prefixes are used only with the units and not alone. For example, 10 μ does not convey anything, it has to be 10 μm, 10 μg.

You can use only one prefix at a time.

SI prefix is not used with the unit °C.

The power to which a prefixed unit is raised applies to the whole unit, including the prefix. For example, 1 km^{2} = (1000 m)^{2} = 10^{6} m^{2}.
Rules for Representing SI Units
The SI units are the result of the attempt of scientists to evolve a common international system of units that can be used globally. It is therefore important that the words and the grammar is logical and defined unambiguously i.e. everyone uses the system of units in the same manner. In order to achieve this objective, a number of grammatical rules have been framed.

While writing the value of physical quantity, the number and the unit are separated by a space. For example, 100 mg is correct but not 100mg.

No space is given between number and °C, degree, minute and second of plane angle.

The symbols of the units are not changed while writing them in plural. For example, 10 mg is correct but not 10 mgs.

The symbols of the units are not followed by a full stop except at the end of a sentence. For example, 10 mg. of a compound is incorrect.

In writing the SI unit obtained as a combination of units a space is given between the symbols. Thus, m s represents metre second while ms stands for millisecond. If the units are written without leaving any space, the first letter may be taken as a prefix.

For numbers less than unity zero must be inserted to the left of the decimal point. For example, 0.928 g is correct but not .928 g.

Symbols of units derived from proper names are represented by using capital letters. When written in full, the unit should not be written in plural. For example, 30.5 joule or 30.5 J is correct but 30.5 Joules or 30.5 j is not correct.

When using powers with a unit name the modifier squared or cubed is used after the unit name. For example, second squared, gram cubed. Area and volume are exception in such cases the qualifier for the power comes first. for example, square kilometer or cubic centimetre.

For representing unit symbols with negative exponent, the use of the solidus (/) sign should be avoided. If used, no more than one solidus should be used. For example, the unit for gas constant (JK^{–1} mol^{–1}) may be represented as J/K mol but not as J/K/mol.