what is inertial & non inertial frame of reference?

(a) Inertial Frame:-

A frame of reference either at rest or moving with a uniform velocity (zero acceleration) is known as inertial frame. All the laws of physics hold good in such a frame.

(b) Non-Inertial or Accelerated Frame:- 

It is a frame of reference which is either having a uniform linear acceleration or is being rotated with uniform speed.

An inertial frame is endowed with the following characteristics:

• All the fundamental laws of physics are valid in inertial frames.
• All the fundamental laws of physics assume the same mathematical shape in all inertial frames.
• Inertial frames are isotopic with respect to mechanical and optical experiments
• The optical experiments performed in an inertial frame in any direction will always yield the same results.