The major difference is the presence or absence of external energy source. In an electrolytic cell, one does not need an outside source of energy like a battery while in a voltaic cell the reaction occurs simultaneously, or without help. The other major difference is that in an electrolytic cell, the flow of electrons is from positive(cathode) to negative(anode). In a voltaic cell the flow of electrons is from negative(anode) to positive(cathode).
