Consider any reaction

The order of the reaction wrt A or B shows how the change in concentration of either A or B will affect the rate of the reaction.

If the rate of reactoin is directly proportional to the concentration of A,then

If the rate of reactoin is directly proportional to the square of concentration of A,then

Thus for the given reaction,

if  

order of the reaction wrt A is a, order of reaction wrt  B is b

The overall order of reaction= a+b [obtained by summing up the individual orders] wrt to the reactants involved

http://www.chemguide.co.uk/physical/basicrates/orders.html