S_(N)1SN1 and S_(N)2SN2 reactions differ on the basis of mechanism. For an S_(N)1SN1 reaction, "bond breaking"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)2SN2 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.