The measure of how long is length and that of how wide is breadth (width). Let's find out how long or how wide a desk is. These two things length and breadth are the physical quantities of the desk. An object can be described by its physical quantities. You can describe a ball by its weight, radius, mass etc.
If you are measuring the radius of a ball, its radius is a physical quantity. Mass of the ball is another physical quantity. You can quantitatively describe the properties of an object by physical quantities.
A number which can describe the physical property of a material or a physical phenomenon quantitatively is called physical quantity.
The most fundamental physical quantities can only be defined if you know how to measure them. Such a definition is called operational definition. If you want to define the fundamental physical quantities such as length, mass and time, you must know how to measure them.
When you measure any physical quantity you must compare it with a particular standard of that physical quantity called unit. You compare with the unit of a physical quantity to measure it.
The base quantities such as length, mass and time are the fundamental ones which do not depend on other quantities. The derived quantities are derived from the base quantities such as area, volume speed etc.
In Physics we often need to describe physical quantities by numerical value or by both the numerical value and direction. The physical quantities with direction are called vectors and those without direction are called scalars. The vectors and scalars distinguish all physical quantities by their distinguishing factor called direction.
