On the AskAboutPHP.com blog eldee takes a look at helpers, plugins and libraries in the CodeIgniter PHP framework including some examples.

With so many published PHP classes and functions, it would be a shame if we couldn't use them in CI. Fortunately CI (like all good frameworks) provides not one but three ways to integrate 3rd code, by using Helpers, Plugins and Libraries. [...] As I found out during the course of my CI project, Helpers, Plugins and Libraries are nothing more than glorified includes. I can pretty much take any 3rd party code and integrate into my application using any of the 3 methods.

His example uses the Google Graph class as each type (helper, library and plugin) with some example code.