CodeIgniter Libraries

Libraries  used to provide common functionality that you want accessed by different parts of your app. They help provide a single point of entry that makes it simple to maintain consistency in business rules. They’re ideally used for creating focused, single-responsibility classes, like Benchmarking, UserObject, Article, etc.

Libraries are not just a file that must contain stand-alone information that can be passed from project to project, though that is the traditional role in CodeIgniter applications. It’s just not the only use for libraries.

It is present in both the system and the application folder.
Again, this is a class. (Note: your own libraries are not automatically included in the CI super-object, so you need to call CI resources in a different way.)
Load it like this: $this->load->library(‘class_name’);
Use it like this: $this->class_name->mainpage();
Notes on syntax:
No need to extend a base class, or for a constructor function.
This is enough:

Conceptually: Intended to hold your own code to extend CI functionality, or to create site-specific functionality.

List of Codeiginter Libraries

(Source :