CodeIgniter is based on the Model-View-Controller (MVC) development pattern. MVC is a software approach that separates application logic from presentation. In practice, it permits web pages to contain minimal scripting since the presentation is separate from the PHP scripting.

Model: The Model represents data structures. Typically model classes will contain functions that help to retrieve, insert, and update information in database.View: The View is the information that is being presented to a user. A View will normally be a web page, but in CodeIgniter, a view can also be a page fragment like a header or footer. It can also be an RSS page, or any other type of "page".Controller: The Controller serves as an intermediary between the Model and the View, and any other resources needed to process the HTTP request and generate a web page.

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

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.

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);