An OLAP cube is a dimensional data structure that answer dimensional query. It store data in a multidimensional way. You can a cube as an extension of the two dimensions relational table.
A cube is defined:
and is generally pre-computed.
The OLAP cube consists of numeric facts called measures which are categorized by dimensions. The cube metadata may be created from a star schema or snowflake_schema of tables in a relational database. Measures are derived from the records in the fact table and dimensions are derived from the dimension tables.