What is the purpose of bootstrap in zend?

I am wondering what is the real purpose of bootstrap in Zend Framework.

What are the methods that go into the Bootstrap class?

Any tutorial links could be helpful... please forgive me if the question is so vague. I am trying to learn Zend but the tutorials and books are skipping steps and they are not so clear in the framework website. All I can learn from the quick start is that it helps to start the session.

Am I right in thinking that the bootstrap runs first before any controller loads? Can I write any methods in bootstrap which I need to load from the beginning? How do I access those bootstrap methods in controller?