The software is written in a language called PHP. From the PHP website, www.php.net, PHP is defined as:
"PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML." Php costs nothing, it is free. PHP 4.3.9 or better is recommended.
A MySQL server is free to download from www.mysql.com (Although your hosting company would have it already installed). The script supports MySQL 3 and MySQL 5
GD Graphics library is a piece of software which is used in conjunction with PHP to give PHP the ability to work with graphics. GD Library 2.0.28 or better is recommended.
In most cases, these things are already installed for you when you sign up for a hosting service. Linux based hosting service is recommended, although the script will work equally well on a Windows based server if the requirements are met.