A data
warehouse is a place where data is stored for archival, analysis and security purposes. Usually a data warehouse is either a single computer or many computers (servers) tied together to create one giant computer system.
Data can consist of raw data or formatted data. It can be on various types of topics including organization's sales, salaries, operational data, summaries of data including reports, copies of data, human resource data, inventory data, external data to provide simulations and analysis, etc.
Besides being a store house for large amount of data, they must possess systems in place that make it easy to access the data and use it in day to day operations. A data warehouse is sometimes said to be a major role player in a decision support system ( 
DSS). DSS is a technique used by organizations to come up with facts, trends or relationships that can help them make effective decisions or create effective strategies to accomplish their organizational goals.
DSS). DSS is a technique used by organizations to come up with facts, trends or relationships that can help them make effective decisions or create effective strategies to accomplish their organizational goals.Data Warehousing Models
There are many different models of data warehouses. Online Transaction Processing, which is a data warehouse model, is built for speed and ease of use. Another type of data warehouse model is called Online Analytical processing, which is more difficult to use and adds an extra step of analysis within the data. Usually it requires more steps which slows the process down and requires much more data in order to analyze certain queries.
In addition to this model, one of the more common data warehouse models include a data warehouse that is subject oriented, time variant, non volatile and integrated. Subject oriented means that data is linked together and is organized by relationships. Time variant means that any data that is changed in the data warehouse can be tracked. Usually all changes to data are stamped with a time-date and with a before and after value, so that you can show the changes throughout a period of time. Non volatile means that the data is never deleted or erased. This is a great way to protect your most crucial data. Because this data is retained, you can continue to use it in a later analysis. Finally, the data is integrated, which means that a data warehouse uses data that is organizational wide instead of from just one department.
Besides the term data warehouse, a term that is frequently used is a data mart. Data marts are smaller and less integrated data housings. They might be just a database on human resources records or sales data on just one division.
Types of Data Warehouses
With improvements in technology, as well as innovations in using data warehousing techniques, data warehouses have changed from Offline Operational Databases to include an Online Integrated data warehouse.
Offline Operational Data Warehouses are data warehouses where data is usually copied and pasted from real time data networks into an offline system where it can be used. It is usually the simplest and less technical type of data warehouse.
Offline Data Warehouses are data warehouses that are updated frequently, daily, weekly or monthly and that data is then stored in an integrated structure, where others can access it and perform reporting.
Real Time Data Warehouses are data warehouses where it is updated each moment with the influx of new data. For instance, a Real Time Data Warehouse might incorporate data from a Point of Sales system and is updated with each sale that is made.
Integrated Data Warehouses are data warehouses that can be used for other systems to access them for operational systems. Some Integrated Data Warehouses are used by other data warehouses, allowing them to access them to process reports, as well as look up current data.