Below are the server requirements for running PyroCMS 2.x. If you aren't sure if you have all of these, PyroCMS will check for them on installation for you.
a HTTP web-server of some description
Apache 2.x, Abyss Web Server, Uniform Server, Zend Community Server, etc. So far PyroCMS only seems to struggle with IIS.
MySQL 5.x or higher
There are plans for other database systems, but for now the newer MySQL version the better.
PHP 5.2.x or higher
PHP 5.2 and PHP 5.3 are both fully supported.
This library needs to be installed on your server with PHP compiled to include it. Usually your package manager will take care of this for you, if not you can get it here (the Libgd wiki is currently unavailable.)
PyroCMS uses these third-party packages to run, but includes them in each release so you do not need to install them yourself: