Question:What are helpers in CodeIgniter? 

Helper files are a collection of functions in a particular group. Helpers can be used within Models, Controller and View files.

Question:What are plug-in for in CodeIgniter? 

Plug-ins work almost identically to helper. A plug-in usually provides a single function, plug-ins are intended to be created and shared by community.

Question:Differentiate between helper and plug-in? 

The main difference is that a plug-in usually provides a single function, whereas a Helper is usually a collection of functions. Helpers are also considered a part of the core system; plug-ins are intended to be created and shared by our community.In loading the difference is
$this->load->plugin(plugin name);
$this->load->helper (file name of the helper);

Question:Why the following files are used in codeigniter?
a) config.php 
b) autoload.php
c) database.php
d) routes.php 

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.

Question:What are codeigniter URL segments? 

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