A double displacement precipitation is a reaction in which two soluble ionic compounds react to form an insoluble precipitate.
Double replacement reactions take the general form:
A⁺B⁻ + C⁺D⁻ → A⁺D⁻ + C⁺B⁻
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
This is a double replacement reaction, because the silver ion and the sodium ion have exchanged partners. It is also a precipitation reaction, because the silver ion and the chloride ion are removed from the solution as solid silver chloride.
Here is a video which shows another example of a double replacement (precipitation) reaction.
Detailed and organized can be on this site http://homepage.smc.edu/walker_muriel/double_displacement_reactions_procedure.htm