A data warehouse is a repository of an organisation's electronically stored data. Data warehouses are designed to facilitate reporting and analysis. 

This classic definition of the data warehouse focuses on data storage. However, the means to retrieve and analyse data, to extract, transform and load data, and to manage dictionary data are also considered essential components of a data warehousing system. Many references to data warehousing use this broader context. An expanded definition for data warehousing includes business intelligence tools, tools to extract, transform, and load data into the repository, and tools to manage and retrieve metadata.

Some of the benefits that a data warehouse provides are as follows:

• A data warehouse provides a common data model for all data of interest, regardless of the data's source. This makes it easier to report and analyse information than it would be if multiple data models from disparate sources were used to retrieve information such as sales invoices, order receipts, general ledger charges, etc.
• Prior to loading data into the data warehouse, inconsistencies are identified and resolved. This greatly simplifies reporting and analysis.
• Information in the data warehouse is under the control of data warehouse users so that, even if the source system data is purged over time, the information in the warehouse can be stored safely for extended periods of time.
• Because they are separate from operational systems data warehouses provide retrieval of data without slowing down operational systems.
• Data warehouses facilitate decision support system applications such as trend reports (e.g., the items with the most sales in a particular area within the last two years), exception reports, and reports that show actual performance versus goals.
• Data warehouses can work in conjunction with and, hence, enhance the value of operational business applications, notably customer realtionship management (CRM) systems

Edit