According to W.H.Inmon...A Datawarehouse is nothing but a
Subject-Oriented
Time-Variant
Non-Volatile
Integreted
storage of data.
Subject-Oriented: Subject-Oriented means it describes about a particular subject like Customer, Sales and Product.
Time-Variant: It means according to time data in datawarehouse changes beacause from time to time data will be loaded into the datawarehouse.
Non-Volatile: Once data is loaded into the datawarehouse it must not be deleted.
Integrated: i.e. data from different sources will be loaded and placed into one place(datawarehouse).
According to Ralph Kimbal, a datawarehouse is a copy of transactional data used for analysis purpose.
Schemas: There are different types of schemas are there for designing datawarehouse. They are
STAR SCHEMA
SNOW-FLAKE SCHEMA