Step 1: Contrast their source. Molecularity comes from the theoretical, balanced elementary step and simply counts how many particles must meet at once. Order comes only from experiment, by seeing how the measured rate changes when you vary each reactant's concentration.
Step 2: Contrast the allowed values. Molecularity must be a small positive whole number, 1 (unimolecular), 2 (bimolecular) or 3 (termolecular), and never zero. Order, in contrast, may be zero, integral or fractional, and can even be negative for some species.
Step 3: Contrast where each applies. Molecularity is meaningful only for a single elementary reaction, whereas order is meaningful for the overall reaction and is controlled by the rate-determining step.
Step 4: Quick example. In \(2N_2O_5 \rightarrow 4NO_2 + O_2\) the balanced equation suggests molecularity two, yet the experimentally observed order is one, showing clearly that the two ideas are not the same.
\[\boxed{\text{Order} = \text{experimental};\ \text{Molecularity} = \text{theoretical}}\]