A database is an organized collection of data. The term originated within the computer industry, and its meaning has been broadened with popular usage so that the European Database Directive (which defines the rights of intellectual property for databases) includes non-electronic databases within its definition.
One possible definition is that a database is a collection of records stored in a computer in a systematic way, so that a computer program can consult it to answer questions. Each entry is, for a better retrieval and sorting, usually organized as a set of elements (facts). The items retrieved in answer to queries become information that can be used for making decisions that might otherwise be much more difficult or impossible to make. The computer program used to manage and query a database is called a database management system (DBMS). Properties and design of database systems are included in the study of information science.
Microsoft SQL is a relational database and its primary language for querying is Transact SQL (T-SQL), which means that in addition to the basic and classic (SELECT type) SQL queries allows more complex things such as altering the program flow (IF command) etc.. Transact SQL was created as a result of the collaboration between Microsoft and Sybase.
Oracle Database (commonly referred to as Oracle RDBMS or simply as Oracle) is an object-relational database management system which is produced and sold by Oracle Corporation.
MySQL is an open source relational database management system (RDBMS), which is the most used in the world since 2008. It is very popular as a choice of databases to create a web site; a large number of web pages are using this type of database.
Postgre is another open source relational database management system (RDBMS). With more than 15 years of active development and a proven architecture it has earned a strong reputation for reliability, data integrity and accuracy. Runs on all major operating systems.