A database is a collection of data stored together as a unit. Databases are useful for storing data and making it available for retrieval. Within the database, data is organized into different tables. Each table has columns and rows. Indexes on tables provide speedy access to data. Information in the database can be retrieved, modified, or deleted using a query language like SQL.
 Types of Databases
There are several types of databases available:
- Flat File Databases
- Relational Databases
- Object Oriented Databases
- Hierarchical Databases
 Flat File Databases
A flat file database is described by a very simple database model where all the information is stored in text files.
Examples of Flat File Databases:
- Any data stored in flat-files
 Relational Databases
Examples of Relational Databases:
- Oracle Database (http://www.oracle.com/database/)
- Microsoft SQL Server (http://www.microsoft.com/sql/)
- MySQL (http://www.mysql.com/products/)
- PostgreSQL (http://www.postgresql.org/)
- Firebird (http://www.firebirdsql.com/)
For more info, see Relational Database Systems.
 Object Oriented Databases
Examples of Object Oriented Databases:
 Hierarchical Databases
The hierarchical model is considered outdated, has too many limitations, and are not frequently used anymore.
Examples of Hierarchical Databases:
- Adabas from Software AG (http://www.softwareag.com/)
- IBM's Information Management System (IMS) (http://www.ibm.com/software/data/ims/)