SN1 and SN2 reactions differ on the basis of mechanism. For an SN1 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 SN2 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.