Data warehousing is a technology that aggregates structured data from one or more sources so that it can be compared and analyzed for greater business intelligence.
Data warehouses are typically used to correlate broad business data to provide greater executive insight into corporate performance.
Data warehouses use a different design from standard operational databases. The latter are optimized to maintain strict accuracy of data in the moment by rapidly updating real-time data. Data warehouses, by contrast, are designed to give a long-range view of data over time. They trade off transaction volume and instead specialize in data aggregation.
Many types of business data are analyzed via data warehouses. The need for a data warehouse often becomes evident when analytic requirements run afoul of the ongoing performance of operational databases. Running a complex query on a database requires the database to enter a temporary fixed state. This is often untenable for transactional databases. A data warehouse is employed to do the analytic work, leaving the transactional database free to focus on transactions.
The other benefits of a data warehouse are the ability to analyze data from multiple sources and to negotiate differences in storage schema using the ETL process.