When two aqueous solutions react, they sometimes form solids in the solution. The solid is called a precipitate. Precipitation reactions occur when the cations of one reactant and the anions of a second reactant found in aqueous solutions combine to form an insoluble ionic solid, that we call a precipitate.
Most precipitates are formed in a double-replacement reaction. A double-replacement reaction is when the ions in two compounds exchange places with each other in an aqueous solution. AX + BY --> AY + BX, where A, B, X and Y are all ions.
An ionic solution is when the ions of a compound have dissociated in an aqueous solution. A reaction happens when you mix two aqueous solutions. This is when you find out if a precipitate will form or not. A precipitate forms if the product of the reaction of the ions is insoluble in water.