In SN1 reactions the rate limiting step is loss of leaving group to give a carbocation. Polar protic solvents have a higher dielectric constant and can stabilize the resulting carbocation species that results from ionization. Secondly, polar protic solvents are often the nucleophiles in these reactions and can be neutralized through deprotonation.
In SN2 reactions the rate limiting step is attack of the nucleophile at the alkyl halide. Using a polar aprotic solvent results in a more free nucleophile (free from hydrogen bonding) resulting in a higher reaction rate than if a polar protic solvent were used.

If the substrate is primary, it will be SN2, since the reaction rate will be faster with a stronger nucleophile.

Cool, thanks for letting me know Sanket. So what could be better about the site?