config.php:The Config class provides a means to retrieve configuration preferences. These preferences can come from the default config file (application/config/config.php) or from your own custom config files.
autoload.php: If we find that we need a particular config file globally, we can have it loaded automatically by the system.
database.php: CodeIgniter has a config file that lets store your database connection values (username, password, database name, etc.). The config file is located at: application/config/database.php
routes.phpRouting rules are defined in application/config/routes.php file. That permits to specify own routing criteria.

CodeIgniter’s URLs are simply segments. These segments are then split up and used to load a particular controller and method. CodeIgniter uses a segment-based approach:
example.com/news/article/my_article

The following is a list of the core system files that are invoked every time CodeIgniter runs:

 Benchmark
Config 
Controller
 Exceptions
 Hooks
 Input
 Language 
Loader
 Log
 Output
 Router
 URI

Unit testing is an approach to software development in which tests are written for each function in your application. CodeIgniter's Unit Test class is quite simple, consisting of an evaluation function and two result functions.

The Input and Security Class comes with a Cross Site Scripting hack prevention filter, which can be run on a per-item basis, or can be set to filter all POST and COOKIE data automatically.