CodeIgniter is a powerful PHP framework with a very small footprint, built for developers who need a simple and elegant toolkit to create full-featured web applications.
CodeIgniter 3 has a 2MB download, including the user guide.
The CodeIgniter User Guide comes with the download. It contains an introduction, tutorial, a number of "how to" guides, and then reference documentation for the components that make up the framework.
CodeIgniter 3 only needs PHP 5.2.4, and plays nicely with almost all shared or dedicated hosting platforms. Many webapps need a database, and CodeIgniter supports the most common, including MySQL.
Use your own coding and naming conventions, with only a few caveats that deal with classname conflicts. CodeIgniter looks to empower you, not shackle you.
CodeIgniter encourages MVC, but does not force it on you.
CodeIgniter consistently outperforms most of its competitors.
CodeIgniter is not trying to be all things to all people. It is a lean MVC framework, with enough capabilities to improve your productivity, while providing for third-party addons/plugins for additional functionality.
Much of the CodeIgniter configuration is done by convention, for instance putting models in a "models" folder. There are still a number of configuration options available, through scripts in the "config" folder.
CodeIgniter comes with a simple, substitution based, templating tool. Addons/plugins are available for most of the full-blown templating engines, if that is what you are used to.
Don't we all want it? CodeIgniter is easy to learn and to get proficient with.