WHAT IS PEAR?

I checked the PEAR HOMEPAGE but still I don't quite understand what the project is about. I am familiar with writing classes and functions. Now, I can not figure out what the deal is. Does Pear consist of a class library that comes with the installation? There is a directory "pear" which is full of rar archives. How does that come into play. Does the standard installation provide SMARTY support or does it need additional configuration? If it does is there some some sort of crash course how to use it? Also an overview of what actually is available would come in handy. Thank you very much. W.