Double Replacement
The exchange of positive ions between two compounds; often results in the formation of solid precipitate, a gas or water.

AY + BX -> BY + AX
AgNO3 + K2CrO4 -> 2KNO3 + Ag2CrO4

Acid + base yields salt + water is neutralization reaction.

You go to a dance with your girlfriend and your friend goes with his girlfriend. Everyone has a terrible time. So you end up with his girlfriend and he ends up with your girlfriend.

Kl + Pb (No3)2-> PbI2 + 2KNO3

We typically talk about acid-base reactions in aqueous-phase environments -- that is, in the presence of water. The most fundamental acid-base reaction is the dissociation of water.