Which databases can be used with Joomla 2.5?

When manually installing Joomla 2.5, at step 4: Database you'll be asked to choose a database type of either MySQLi or MySQL. It doesn't matter if you choose MySQLi or MySQL, you will still be using a MySQL database. MySQL is in fact required in order to use Joomla, and there is currently not an option to use any other database type.

Question: Which databases can be used with Joomla 2.5?
Answer: MySQL

MySQLi vs. MySQL for Joomla 2.5

MySQLi and MySQL are both php extensions that allow Joomla 2.5 to interact with the database. MySQLi stands for MySQL Improved. Not only is it recommended by Joomla, but php's own website even suggests using MySQLi if you are using MySQL versions 4.1.3 or higher (which most servers are). If you'd like to learn more above the differences between MySQLi and MySQL, please see this page.