Galvanic cells are constructed such that a spontaneous redox process occurs within a closed system to harness the flow of electrons to do work. Electrolytic cells are those that require an input of energy to force redox reactions to occur. The input of an electric current to generate reactions is known as electrolysis.
Chemical cells are composed of two electrodes, where the reactions occur, and the solutions in which the electrodes are placed. The solutions in which the electrodes are placed are electrolytic; charge can move through them due to the presence of dissociated ion salts. If the electrodes are in separate containers, the circuit is completed by the addition of a salt bridge joining the two cells which allows for the balancing of change between the two containers but prevents the mixture of the two electrolytes. The electrode at which the reduction reaction occurs is the cathode, while the electrode at which the oxidation reaction takes place is the anode. The sign conventions for galvanic and electrolytic cells are as follows:
