CodeIgniter is a PHP framework, what that means is that CodeIgniter is based on PHP and it is a set of files that make a website, it is a template to making a website, some thing like a mould. So, we have a mould to a website, but this mould is not complete, we need to sharpen the mould to our likings.
CodeIgniter follows the MVC approach of organizing code. MVC means Model-View-Controller. The Model deals with database, View deals with the look or HTML, Controller coordinates the model and view.
For anyone wanting to start CodeIgniter to build a website, can head over to this website. When you finished downloading, there will be files.
We will mostly deals with application/controllers, application/models, and application/views.