To autoload resources, open the application/config/autoload.php file and add the item you want loaded to the autoload array.
The database.php file contains all the information to connect to a database. A Database is required for most web application programming. CodeIgniter supported databases are MySQL (4.1+), MySQLi, MS SQL, Postgres, Oracle, SQLite, and ODBC
This config.php file are located under application/config filder or your own custom configuration file. This class is initialized automatically by the system so there is no need to do it manually.
The URL Helper file contains functions that assist in working with URLs. The helper('url') class is used to load the url helper class
CodeIgniter is fairly restrictive regarding which characters for URL. The "routes.php" file lets you remap URI requests to specific controller functions.
This tutorial will help you to learn about "how to install the codeigniter", "how to increase the security for application folder"
CodeIgniter is an Application Development Framework - a toolkit - for people who build web sites using PHP. Its goal to enable you to develop projects much faster than you could if you were writing code from scratch