MySQL is an open source RDBMS that relies on SQL for processing the data in the database. MySQL provides APIs for the languages C, C++, Eiffel, Java, Perl, PHP and Python. It is most commonly used for web and embedded applications and has become a popular alternative to proprietary database systems because of its speed and reliability.
All newly created databases now use MySQL5. If you have an database that was created some time ago, this database may be a MySQL4 database however MySQL4 is being phased out. All new databases created will utilize MySQL5 only.