 product, A combines with C, while B combines with D. Because the replacement occurs in two places, it is referred to as a double replacement. Double replacement reactions are also called metathesis or double displacement reactions. Usually, in these reactions, when combining aqueous solutions, a solid product is also formed. This product is referred to as a precipitate, and the reaction is called a precipitation reaction.
For example, take a look at this reaction:
AgNO3 (aq) + NaCl (aq) —— AgCl (s) + NaNO3 (aq)
When an aqueous solution of Silver Nitrate is mixed with an aqueous solution of Sodium Chloride, it results in the formation of a Sodium Nitrate solution, while Silver Chloride precipitates out. One can easily see that elements have been replaced in both the reacting compounds.
To summarize, in single replacement reactions, the more reactive element takes the place of a less reactive compound. On the other hand, in double replacement reactions, both of the reacting compounds swap partners and end up forming entirely new compounds!