Ralph Kimball the co-founder of the data warehousing concept has defined the data warehouse as a "a copy of transaction data specifically structured for query and analysis".
Data warehousing is closely linked to OLAP - Online Analytical Processing - which relates to complex analysis of (usually aggregated, or denormalized) data in a data warehouse.
In contrast, OLTP refers to Online Transaction Processing. An OLTP database is used for day-to-day business operations and is therefore well-normalized.
"Datawarehouse vs OLTP." Diffen.com. Diffen LLC, n.d. Web. 23 Jul 2014. < http://www.diffen.com/difference/Datawarehouse_vs_OLTP >