CodeIgniter is an open source web application framework. It's used for creating dynamic web sites and RIAs (Rich Interface Applications) with PHP, MySQL and XML.
Of all the PHP framework libraries, CodeIgniter is one of the fastest and lightest. It's clear documentation and no-need for command-line access is what first caught our eye and drew us in. We love using CodeIgniter for quick, smaller scale projects especially for the server-side back end of our Flash apps and games.
We find CodeIgniter is also great for updating and structuring older sites and projects that have "legacy" code and MySQL databases, without needing to start all over from scratch to adhere to the strict coding and database table requirements of other frameworks like CakePHP.