Introduction:  Measurement of
any physical quantity involves comparison with a certain
basic, arbitrarily chosen, internationally accepted
reference standard called unit.
Magnitude:  The number of
times, a standard quantity is present in the physical
quantity, is known as the ‘magnitude’ of the
physical quantity.
Fundamental Quantity:  A
physical quantity which is independent of any other
quantity is a fundamental quantity.
Fundamental Unit:  The unit
of a physical quantity which is independent of any other
quantity is called a fundamental unit.
Complementary Fundamental Units: 
Units of angle (degrees) and solid angle.
Derived Quantities:  The
physical quantities which can be derived from other
physical quantities are called derived quantities.
All other quantities except the
fundamental quantities are derivable. The units of these
quantities are also derivable from the fundamental units
and are called derived units.
Derived Units:  The units of
physical quantities which can be expressed in terms of
fundamental units are called derived units.
For example, area, pressure, density
and speed are derived quantities and their units – square
meter, Pascal kilogram metre^{3} and meter
second^{1} are derived from the fundamental
units.
SYSTEMS OF UNITS
To measure the fundamental quantities
length, mass and time there are three standardized systems
of units. They are:

CGS
(Metric)

FPS
(British)

MKS
System.
Their units are as shown in the table
below:
System 
Length 
Mass 
Time 
FPS
CGS
MKS 
Foot
Centimeter
Meter 
Pound
Gram
kilogram 
Second
Second
Second 
C.G.S. System: In this system
the unit of length is centimeter; the unit is mass
of gram, and the unit of time is second.
F.P.S. System: In the F.P.S.
system, the unit of length is foot, the unit of
mass is pound and the unit of time is second.
M.K.S. System: In this system,
the unit of length is meter, the unit of mass is
kilogram and the unit of times is second.
Coherent System of Units:  A
coherent system of units is a system based on a certain
set of basic (or fundamental) units from which all derived
units may be obtained by simple multiplication or division
without introducing any numerical factors. S.I. system of
units is a coherent system of units for all types
of physical quantities.
S.I. Units:  S.I. unit is the
short name of “System International de units “which
means “International system of units”. It is an
extension of M.K.S. system. Derived S.I. Units
