What is extreme programming and when it is using

I am new to programming and I try to research as much as possible in this field. And once I came across to this expression: "Extreme and pair programming". Pair programming is an easy term, and I found quite clear documentation about this. But extreme programming... I found some articles about it, but explanation wasn't so well. All I understood that extreme programming is an Agile development framework. But why I must use that, what is difference between this and another types of programming styles?

Can anyone explain me what is extreme programming language very clearly?