Using CodeIgniter Libraries
All of the available libraries are located in your system/libraries folder. In most cases, to use one of these classes involves initializing it within a controller using the following initialization function:
$this->load->library('class name');
Where class name is the name of the class you want to invoke. For example, to load the validation class you would do this:
$this->load->library('validation');
Once initialized you can use it as indicated in the user guide page corresponding to that class.
Creating Your Own Libraries
Please read the section of the user guide that discusses how to create your own libraries