database refers to a collection of data that is multidimensional in the sense that internal links between its entries make
the information accessible from a variety perspectives.
flat file: a traditional file system is a one-dimensional storage system,meaning that it presents its information from a
single point of view.
schema is a description of the entire database structure that is used by the database software to maintain the database.
subschema is a description of only that portion of the database pertinent to a particular user's needs.