Kinetics :  Order and Molecularity
Molecularity - The number of molecules consumed in a chemical reaction

Step 1: N2O5 NO2 + NO3
          Step 2: NO2 + NO3 NO2  + NO + O2
Step 3: NO + NO3 2 NO2

Unimolecular - When a single molecules is consumed.  e.g. Step 1 in the above reaction.
Bimolecular - When two molecules are consumed.  e.g. Steps 2 and 3 in the above reaction.
Order - Describes the relationship between the rate of a step in a chemical reaction and the concentration of one of the reactants consumed in that step.
Rate = k[N2O5]

The above reaction is first order with respect to [N2O5] and first order overall.