CodeIgniter is a Model-View-Controller (MVC) framework. This means that it follows a specific pattern for organizing and structuring your code.

The Model represents the data and business logic of your application. It is responsible for interacting with the database and performing tasks such as inserting, updating, and deleting data.

The View is the user interface of your application. It is responsible for displaying the data to the user and for handling the user input.

The Controller is the glue that binds the Model and the View together. It receives input from the user, communicates with the Model to retrieve or update data, and then passes the data to the View to be displayed.

In CodeIgniter, the Model and View files are optional. You only need to create a Controller and a View file for each page of your application. The Controller contains the logic of your application, and the View contains the HTML code that is displayed to the user.

CodeIgniter also has a number of libraries and helpers that you can use to perform common tasks such as form validation, file uploading, and sending email.

Here’s a general overview of the application architecture in CodeIgniter:

application/
    config/
        config.php
        database.php
    controllers/
        Welcome.php
    models/
    views/
        welcome_message.php
system/
    core/
    database/
    libraries/
    helpers/

The config directory contains configuration files for your application. The controllers directory contains your Controller classes. The models directory is where you would place your Model classes, if you choose to use them. The views directory contains your View files. The system directory contains the core CodeIgniter files and libraries.