what is the diffrence between polar aprotic solvent and non polar solvents???...and what r their effects on nucleophilicity??

If the solvent is aprotic ( it is not a hydrogen bond donor), the direct relationship between nucleophilicity & basicity holds. For example, both the nucleophilicities & basicities of the halogens decrease with increasing size in an aprotic solvent such as dimethylformamide.

If the solvent is protic ( it is a hydrogen bond donor such as water, alcohol,etc,), the relationship between the basicity & nucleophilicity becomes inverted : as basicity decreases nucleophilicity increases. Thus, iodide ion, which the weakest base of the halogen family, is the poorest nucleophile in an aprotic solvent and the best nucleophile in a protic solvent.

When a negatively charged species is placed in a protic solvent, the solvent molecules arrange themselves so that their partially  positively charged hydrogens point toward the negatively charged species. An aprotic solvent does not have a partially positively charged hydrogen.

The interaction between the ion & the dipole of the protic solvent is called as ion-dipole interaction. The change from a direct relationship between basicity & nucleophilicity in an aprotic solvent to an inverse relationship in a protic solvent results from the ion-dipole interactions between the nucleophile and the protic solvent. This occurs because atleast one of the ion-dipole interactions must be broken before the nucleophile can participate in an SN2 reaction. Weak bases interact weakly with protic solvents; strong bases interact more strongly because they are better at sharing their electrons. It is therefore easier to break the ion-dipole interactions between an iodide ion and the solvent than between the more basic fluoride ion and the solvent, because the latter is a stronger base.