The reaction in which there is an exchange of ions between the reactant takes place is called double displacement reaction.
A and C are Cations (Positive lons)
B and D are Anions (Negative lons)
Double Displacement Reaction
For examples:
$$
\mathrm{Na}_{2} \mathrm{SO}_{4}+\mathrm{BaCl}_{2} \rightarrow \mathrm{BaSO}_{4}+2 \mathrm{NaCl}
$$
Sodium Barium Barium Sodium
sulphate chloride sulphate chloride
$$
\mathrm{AgNO}_{3}+\mathrm{NaCl} \rightarrow \mathrm{AgCl}+\mathrm{NaNO}_{3}
$$
Silver Sodium Silver Sodium
nitrate chloride chloride nitrate
