#S_(N)1# and #S_(N)2# reactions differ on the basis of mechanism. For an #S_(N)1# reaction, #"bond breaking"# is the rate determining step, and the reaction is conceived to proceed thru the intermediacy of a carbocation. On the other hand, for #S_(N)2# reactions, bond making is the rate determining step, and the reaction proceeds thru a concerted mechanism in which the geometry of the substrate undergoes inversion upon bond-making. All of this will be explained much better in your organic text.