Differences between Sn1 and Sn2 reactions

--> Sn²: 

It is a one-step mechanism which is considered to be Sn² where two species are involved in the rate determining step. Both species account for the overall rate of the reaction where the rate equation is: rate = k[R-X][OH^-] where X is the halogen. The overall order for this reaction is 2. Sn² occurs in reactions where primary halogenoalkanes are present, forming alcohols. Even though the reaction occurs in one step only, there is a transition state occuring in between, where there is bond breakage and bond formation, in order to remove the halogen group, and add the hydroxide ion (OH-). 

--> Sn¹:

In contrast, the Sn¹ reaction occurs in two steps. Only one species is involved in this reaction, and the first step of the reaction is where the bond between the carbon atom and the halogen breaks. This is the rate determining step of the reaction, which is the slowest step in the reaction. Therefore, the rate equation includes only the halogenoalkane: rate = k[R-X] where X is the halogen. The overall rate for this reaction is 1. Sn¹ occurs where tertiary halogenoalkanes are involved, to form tertiary alcohols. As said above, it occurs in two steps. the first step is where the C-X bond breaks, and this forms a tertiary carbocation. Moving to the second step, the OH^- group attacks the carbocation, forming the alcohol. 

In conclusion, the main differences of these two reactions are the different types of halogenoalkanes present,  the steps in the mechanism, their rate equations and the overall reaction orders.