Galvanic Cells and Voltages

With any given reaction that occurs in a galvanic cell, as long as the metals are of different elements, there will be an oxidiser and a reducer, concurrently there will be two half equations demonstrating the reactions that occur.Everything is compared to the hydrogen electrode, the potential of hydrogen is 0V.

Zn2+(aq) + 2e- ⇌ Zn(s) -0.76V
Cu2+(aq) + 2e- ⇌ Cu(s) 0.34V

These equations show the reducing voltages.
However Zinc being more reactive is oxidising hence the equation moves the other way producing a positive voltage.

Zn(s) ⇌ Zn2+(aq) + 2e- 0.76V
Cu2+(aq) + 2e- ⇌ Cu(s) 0.34V

The overall equation:

Zn(s) + Cu2+(aq) → Zn2+(aq), + Cu(s),, 0.34V

OIL RIG and AN OX are two little mnemonics to help remember which part of the cell functions as the reducer.

Oxidising is losing electrons and reducing is gaining.
The oxidising electrode is the anode, the negative electrode.

The electrolyte for the anode can be referred to as the anolyte, whilst the electrolyte for the cathode can be referred to as the catholyte.