This document compares the Yii web application framework to CodeIgniter. It states that Yii is a more fully-featured framework for developing complex web applications, while CodeIgniter is better suited for simpler projects. Some key advantages of Yii mentioned include its code generation abilities, form handling, integration of jQuery, support for internationalization, and handling of database relations through features like lazy loading. Overall, the document argues that Yii's more complete set of features make it a better choice than CodeIgniter for most web development projects.