Question: what are frameworks?

Hi there, I've heard of frameworks, but I don't quite know what they are used for. I've done a little search on the internet, but even though I've been able to find different PHP frameworks, I'm not quite sure what they offer, or in what they differ, or why I shouldn't just use PHP as it is. Can someone give a little bit of explaination? Thanks! -- --- Contact info: Skype: parham-d MSN: fire_lizard16 at hotmail dot com email: parham90 at GMail dot com