A galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy. In this device the Gibbs energy of the spontaneous redox reaction is converted into electrical work which may be used for running a motor or other electrical gadgets like heater, fan, geyser, etc. Daniell cell is one such cell in which the following redox reaction occurs.

Zn(s) + Cu²⁺(aq) → Zn²⁺ (aq) + Cu(s)

This reaction is a combination of two half reactions whose addition gives the overall cell reaction:

(i) Cu²⁺ + 2e^– → Cu(s) (reduction half reaction) 

(ii) Zn(s) → Zn²⁺ + 2e^– (oxidation half reaction) 

These reactions occur in two different portions of the Daniell cell. The reduction half reaction occurs on the copper electrode while the oxidation half reaction occurs on the zinc electrode. These two portions of the cell are also called half-cells or redox couples. The copper electrode may be called the reduction half cell and the zinc electrode, the oxidation half-cell.

Which electrode is used to find the the reduction potential of half cells? 

Standard Hydrogen electrode

According to convention, a half-cell called standard hydrogen electrode represented by Pt(s)|H2(g)|H⁺(aq), is assigned a zero potential at all temperatures.

At 298 K the EMF of the cell,

Standard hydrogen electrode || Second half-cell constructed by taking standard hydrogen electrode as anode (reference half-cell) and the other half-cell as cathode, gives the reduction potential of the other half-cell.