Some important types of electrodes which are frequently used in electrochemical cells are described as follows:    

Metal-metal ion electrode: such type of electrodes includes a metal rod dipped in the solution of its own ions. Some examples areZn/Zn²⁺, Ag/Ag⁺, Cu/Cu⁺ etc.
    
Amalgam electrodes: these are similar to metal-metal ion electrodes, but here the metal is used in the form of its amalgam with hg. Amalgam is formed to modify the activity of metal. (An-Hg)/Zn²⁺ is one of the common examples of this type.
    
Gas electrodes: such electrodes involve inert metal such as platinum dipped in the solution containing ions of the gaseous element. The arrangement is made in such a manner that gas and its ions are brought in contact at the surface of the inert metal. Some examples along with their electrode reactions are as follows:

Hydrogen electrode   :    Pt, H₂(g)/H⁺(aq)
H₂    2H⁺ + 2e^-              or 2H⁺ + 2e^-    H₂
Chloride electrode    :    Pt, Cl₂(g)/Cl^-(aq)
2Cl^-    Cl₂ + 2e^-            or Cl₂ + 2e^-    2Cl^-
Bromine electrode     :   Pt, Br2(g)/Br-(aq)
2Br^-    2Br₂ + 2e^-          or Br₂ + 2e^-    2Br^-
    
Metal-metal insoluble salt-salt anion: in such electrodes, the metal and some sparingly soluble salt of metal are dipped in aqueous solution of some other soluble salt whose anion is same as that of sparingly soluble salt of the metal. One of the common examples is Ag-AgCl(s)/KCl(aq). Here, silver and sparingly soluble salt of Ag, i.e. AgCl is put in aqueous solution of KCl. The concentration of Ag⁺ions in KCl solution is controlled from the knowledge of K_sp of AgCl as follows.

 

The electrode reactions are as follows:

Ag(s) + Cl^-(aq)    AgCl + e^-             oxidization
    
AgCl(s) + e^-    Ag(s) + Cl^-(aq)      reduction 

Another important example of this type of electrode is calomel electrode: Hg-Hg₂Cl₂(s)/KCl(aq), whose electrode reactions are:

2Hg(l) + 2Cl^-(aq)    Hg₂Cl₂(s) + 2e^-      oxidization  
  
Hg₂Cl₂(s) + 2e^-    2Hg(l) + 2Cl^-(aq)       reduction
    
Redox electrodes: these electrodes include a platinum wire dipped in a solution of a mixture of two salts of the same metal having different oxidation states. Some ordinary examples are listed below:
 
Pt, Fe₂+/Fe²⁺ Fe²⁺    Fe²⁺ + e^-

Or, Fe²⁺ + e^-    Fe²⁺

Pt, Sn²⁺/Sn⁴⁺ Sn²⁺    Sn⁴⁺ + e^-

Or, Sn⁴⁺ + 2e^-    Sn²⁺