Influence of protic and aprotic solvents on SN reactions
As you know, $\mathrm{S_N1}$ and $\mathrm{S_N2}$ reactions have different mechanisms. $\mathrm{S_N1}$ occurs in 2 steps: formation of a carbocation and addition of the nucleophile. However, $\mathrm{S_N2}$ has a one-step mechanism where the addition of the nucleophile and the elimination of the leaving group happen simultaneously. 

A protic solvent would participate in hydrogen bonding with the nucleophile since it can easily share one proton. A solvent shell is thus formed around the nucleophile, making him less able to undergo a $\mathrm{S_N2}$ reaction.