We've looked at the way CI builds up one super-object to make sure that all the methods and variables you need are automatically available to you without you having to manage them and worry about their scope.
CI makes extensive use of assignment by reference, instantiating one class after another and linking them all together so that you can access them through the super-class. We've also looked at how you can write your own classes and still have access to the CI framework.
Also, we looked at a few problems that can arise, particularly if you're not used to OO programs, and seen a few solutions to them. Lastly we looked at how we can extend core classes and even the controller class.
If you want to go deeper into extending CI it is recommended to take a look at these links, they will be of great help: