Step 1: Define order of reaction.
Order of reaction is the sum of the powers of the concentration terms in the experimentally determined rate law. For rate $= k[A]^m[B]^n$, the order is $m + n$. It is an experimental quantity.
Step 2: Define molecularity.
Molecularity is the number of reacting species (atoms, ions, or molecules) that collide simultaneously in a single elementary step of the reaction mechanism. It is a theoretical concept.
Step 3: First difference.
Order is determined experimentally from rate data and can be zero, a fraction, or any non-negative number. Molecularity is derived from the mechanism and can only be a positive integer (1, 2, or 3); it can never be zero or fractional.
Step 4: Second difference.
Order applies to both elementary and complex (multi-step) reactions overall. Molecularity is defined only for individual elementary steps of a mechanism, not for the overall complex reaction.