Define Order and Molecularity?

The order of reaction with respect to a reactant is defined as the power (exponent) of the concentration of the reactant to which the rate of reaction is directly proportional. The overall order of reaction is the sum of all the exponents of all the reactants. 

Molecularity on the other hand is defined as the number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction.

Also, the order of a reaction is the same as its molecularity but the converse is not true.