There are following differences in the order and molecularity of a reaction :
(1) The order of a reaction is an experimental quantity while molecularity is a theoretical quantity.
(2) Order can also be zero or fractional whereas molecularity is always an integer.
(3) The mechanism of reaction is known from the order not from molecularity.
(4) Order of a reaction is applicable for both primary and complex reactions but molecularity of reaction is applicable only for primary reactions. The molecularity of complex reactions has no meaning. In complex reactions, the order is given by the slowest term and generally, the molecularity and order of the slowest term are the same.
(5) Order depends on temperature, pressure or reaction conditions but not molecularity.
(6) Order is related to concentration whereas molecularity is related to the number of molecules.