What is SQL?
SQL stands for Structured Query Language and is used to communicate with a database. According to American National Standards Institute, it is the standard language for relational database management systems. SQL statements are used to perform tasks such as update data on a database, or retrieve data from a database. Some common relational database management systems that use SQL are: Oracle, Sybase, Microsoft SQL Server, Access, Ingres, etc. The standard SQL commands such as "Select", "Insert", "Update", "Delete", "Create", and "Drop" can be used to accomplish almost everything that one needs to do with a database.
MySQL is an open source relational database management system (RDBMS).
PostgreSQL, is an object-relational database management system (ORDBMS) with an emphasis on extensibility and standards compliance.
SQLite is a RDBMS and in contrast to many other database management systems, it is not a client–server database engine. Rather, it is embedded into the end program.
SQL Server is a RDBMS developed by Microsoft. As a database server, it is a software product with the primary function of storing and retrieving data.