The Answer: Database views; Database views are mechanisms that restrict access to the information that a user can access in a database.Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 35.
Wikipedia has a detailed explantion as well: In database theory, a view is a virtual or logical table composed of the result set of a query. Unlike ordinary tables (base tables) in a relational database, a view is not part of the physical schema: it is a dynamic, virtual table computed or collated from data in the database. Changing the data in a table alters the data shown in the view.
Views can provide advantages over tables;
They can subset the data contained in a table
They can join and simplify multiple tables into a single virtual table
Views can act as aggregated tables, where aggregated data (sum, average etc.) are calculated and presented as part of the data
Views can hide the complexity of data, for example a view could appear as Sales2000 or Sales2001, transparently partitioning the actual underlying table
Views do not incur any extra storage overhead
Depending on the SQL engine used, views can provide extra security.
Limit the exposure to which a table or tables are exposed to outer world
Just like functions (in programming) provide abstraction, views can be used to create abstraction. Also, just like functions, views can be nested, thus one view can aggregate data from other views. Without the use of views it would be much harder to normalise databases above second normal form. Views can make it easier to create lossless join decomposition.