What is a data warehouse?

From wiki:

A data warehouse is a repository of an organization's electronically stored data, designed to facilitate reporting and analysis 1.

This definition of the data warehouse focuses on data storage. However, the means to retrieve and analyze data, to extract, transform and load data, and to manage the data dictionary are also considered essential components of a data warehousing system. Many references to data warehousing use this broader context. Thus, 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.

For non technical guys the best is to describe it as "Huge ammount of data stored in a specialized computer system. Data is usually related to some specific domain and whole system is designed to be fast and optimized for some special tasks. Data stored in data warehouses are mostly used for analysis or in decision making processes."

Not sure if this is enough:) There is a lot of references to this subject in the Internet but if somebody asked me for a quick definition I would use something similiar to that I wrote above.