Units are of two types:
(1) Fundamental Units(2) Derived Units
(1) Fundamental units:
The units selected for measuring the fundamental quantitits (mass,length and time) are called fundamental units.All the physical quantities can be expressed in terms of these fundamental units.
For measuring mass,length and time there are independent unit such as kilogram (kg),metre (m) and (s) hence kilogram (kg), metre (m) and second (s) are called fundamental units.
Unit of mass: Mass of a body is the quantity of matter contained in the body.It is an essential property of material body and hence can never be zero.It is also not affected by the presence of other bodies near the body.The internationally accepted unit of mass is kilogram(kg).
The general conference of weight and measures defined kilogram as the mass of platinum-irridium cylinder kept at the international bureau of weight and measures at sevre near paris,france.
Unit of length: Length of an object may be defined as the distance of separation between its two ends.Internationally accepted unit of length is metre (m).General conference of weight and measuress defined metre as the distance between two lines marked as a platinum-irridium rod-kept at a constant temperature of 273.16 k at international bureau of weight and measures at sevres near paris,france.
Unit of time:According to einstein,time is simply what a clock reads.
internationally accepted unit of time is second (s).
Since measurement of mass,length and time is independent and cannot be resolved into any other units,hence units of mass,length and time are fundamental units.
(2)Derived unit:The units of all physical quantities which can be expressed in terms of fundamental units are called derived units.In other words,the units of derived physical quantities are called derived units.
For example, area=length*length=(length)2
speed=distance/time
acceleration.=mass*length/(time)2
Thus the units of area,velocity,acceleration,and force are derived from the fundamental units of mass, length and time etc.