Imagine trying to run a business by looking only at the cash register screen right this second. You see a sale, but you have no idea what sold best last month, which customers are most loyal, or whether your latest promotion actually worked. A data warehouse is the memory that lets an organization answer those big-picture questions. In this chapter, we explore what a data warehouse really is, why it is built so differently from everyday databases, and how raw operational data gets cleaned, reshaped, and loaded into it through a pipeline called ETL.