The sum of powers of the concentration term in the rate equation of a chemical reaction is called Order of reaction.
Order of reaction obtained from the rate law equation which is obtained experimently.
Let us consider a general chemical reaction-
nA + mB = Product
then, the rate of reaction = k[A]x [B]y
The value of x and y may or may not be equal to n and m respectively because the value of x and y are determined experimently.
If the reaction takes place in more than one steps, then, the slowest step is rate determining step.
The value of order of reaction may be positive, negative, zero, fraction or integer. The order of reaction can never be more than or equal to the molecularity of the reaction.