Two reacting ionic compounds exchange partners to form two new products.
Double replacement reactions take the form:
A⁺B⁻ + C⁺D⁻ → A⁺D⁻ + C⁺B⁻
AgNO₃ + NaCl → AgCl + NaNO₃
is a double replacement reaction. The silver ion and the sodium ion exchanged partners.
Pb(NO₃)₂ + 2 KI → PbI₂ + 2 KNO₃
is a double replacement reaction, because the lead ion and the potassium ion have traded partners.