Step 1 (order): Order tells us how the speed of a reaction actually depends on concentration. Write the rate law from experiment, add up the exponents of the concentration terms, and that sum is the order. Because it comes from experiment, order can be 0, 1, 2, or a fraction like 1/2 or 3/2.
Step 2 (molecularity): Molecularity comes from the mechanism, not the stopwatch. It counts how many particles must meet in one elementary collision. Since you cannot have a fraction of a molecule colliding, molecularity is a small whole number (1, 2 or at most 3) and is meaningless for a multi-step overall reaction.
Step 3 (same value example): Consider \(2NO + O_2 \rightarrow 2NO_2\), treated as elementary. Three molecules meet, so molecularity = 3; experiment gives rate = k[NO]2[O2], so order = 2 + 1 = 3. They agree.
Step 4 (different value example): Take the inversion of cane sugar, \(C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+} C_6H_{12}O_6 + C_6H_{12}O_6\). Two species react, so molecularity = 2; but water is present in huge excess and its concentration barely changes, so the measured rate depends only on sugar, order = 1 (pseudo first order).
Conclusion: Order is experimental and can be fractional or zero; molecularity is theoretical and always a whole number for an elementary step. For a single-step reaction they are equal, but for many real reactions (e.g. hydrolysis in excess water) they differ.