1. Quantity Those things we can measure is called quantity.
There are tow types of quantity.
1. Scalar quantity
2. Vector quantity

2. Scalar Quantity
A quantity that has only magnitude and no direction is called a scalar quantity. For example, the length of a scale, area of a surface, volume & mass of a body, density, speed etc are scalar quantities.
Vector Quantity
A quantity that has both magnitude & direction is called a vector quantity. For example, displacement, velocity, acceleration, force etc. are vector quantities.

3. Unit
Unit: If we want to measure any quantity then it is possible to take limited amount of that quantity as base and measure that quantity using the base. This base is known as unit.
In other words, a specific measure of physical quantity such as length, mass, time etc., specific multiply of which are used to express magnitudes of that physical quantity.
There are two types of unit
Fundamental Unit :
Derived Unit:.

4. Unit I) Fundamental Unit : Doesn’t depend on any other unit. Example:
Unit of mass
Unit of time
Unit of length
II) Derived Unit : Depends on other units.
 Volume = length  height  width
= 1m  1m  1m
= 1m3

5. Systems of Units: Foot-Pound-Second System or British System(F.P.S):
Unit of length : Foot
Unit of Mass : Pound
Unit of Time : Second
Centimetre-Gramme-Second System or French System(C.G.S):
Unit of length : Centimetre
Unit of Mass : Gramme
Unit of Time : Second
Metre-Kilogramme-System or System(M.K.S):
Unit of length : Meter
Unit of Mass : Kilogramme
Unit of Time : Second

6. Base quantity Base Unit Name Symbol Time t second s Length l meter m
International Systems of Units (S.I Unit):
The International system of Units (SI) is a system of measurement that has been agreed internationally. The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent, as given in Table below.
Base quantity
Base Unit
Name
Symbol
Time t
second s
Length l
meter m
Mass
kilogram kg
Temperature
T, 
Kelvin K
amount of substance
mole
mol
luminous intensity
candela cd
Electric current I
ampere A

7. Different Unit
Meter: The meter is the length of the path traveled by light in vacuum during a time interval of 1/ of a second.
Kilogram: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram
Second: The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

8. Different Unit
Ampere: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 Newton per meter of length.
Kelvin: The Kelvin, unit of thermodynamic temperature, is the fraction 1/ of the thermodynamic temperature of the triple point of water

9. Different Units
Mole: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in kilogram of carbon 12; its symbol is "mol."
When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Candela: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.