A database system optimised for reporting, particularly in aggregate. Often, but not always implemented using a star schema.
A Data Warehouse is a specialised database system that is optimised for reporting or at least easy extraction of data. One important point is that the data in a data warehouse is usually loaded from external systems, and the database design has significant differences to those used for transactional systems. Data warehouse systems often have several characteristic features:
Use of star schemas with central fact tables joining to dimension tables. These facilitate fast aggregate reporting and simple query plans. Sometimes other designs such as snowflake schemas or relatively normalised operational data store databases are employed.
Storage of historical data - often data warehouse systems are used for analytical queries that examine historical data, or trends in aggregate figures over time.
ETL processes to load the data from external sources. Data warehouse systems often function to aggregate data from multiple sources.
Conformed data - data from multiple sources is often transformed into a common format, allowing data from multiple sources to be queried at the same time.