141well the difference is really confusing!!
but the general trend is if the basicity increases the nucleophilicity increases but there r exceptions based on mobility and charge separation..for eg. take F- andI-..F- is highly basic in aqueous medium due to which the H+ surroud the F- and make it almost immobile and decreases its nucleophilic strength as it becums more
stable and hence I- becomes a better nucleophlie in dat case as it isnt so stabilised by the H+ ions....so i hope i cud give u general idea abt the highly confusing topic