@gayathri
the thing that u need to understand here is that , as far as dissociation is concerned its confined within the electrolyte only , and when the electrolyte is being added into a solution then it will form the ions , at that time it will called as ionisation of electrolyte , "
if any substances which donot form ions by dissolving , then that can be called as dissociation ,,
HOPE IT CLEARS YOUR DOUBT ..