making sense of all the xml extensions in php 5

I am trying to make sense of the following 4 xml extensions distributed with
php 5

ext/libxml <- ??
ext/simplexml {experimental)
ext/xml
ext/dom {experimental)

Dom extension's name is self describing. but about the others ?

As I understand, simplexml is a simpler-to-use alternative to xml
extension. what about that libxml ? whats it doing there..is it an
extension truly ? If its not a extension its existence in the "ext"
directory is confusing. I couldnt find the purpose of libxml mentioned
anywhere.

Could someone please explain ?

-- Roshan