In a double displacement reaction, the ions are exchanged between the compounds when two solutions containing soluble salts are combined. One of these compounds settles to the bottom of the container when it has solid formed and is insoluble in water. This substance is known to as a precipitate, and the associated reaction is known as a precipitation reaction.
Several examples of precipitation reactions are
Na2CO3(aq)+CaCl2(aq) → CaCO3(s)↓+2NaCl(aq)
NaCl (s) + AgNO3 (aq) → AgCl (s)↓ + NaNO3 (aq)