LIVE: Live pages can be seen by anybody and by the search engines.
DRAFT: Draft pages can only be seen by logged in CMS users. Draft pages can not be seen by non-logged in visitors, nor by the search engines.
HIDDEN: Hidden pages can be seen by non-logged in visitors. They can be accessed by anyone with a link or the page URL. Hidden pages can not be seen by the search engines, as standard*.
* Hidden pages can not be seen by the search engines, please note that if you are moving a page from 'Live' to 'Hidden', occasionally search engines might not detect the change. Google is very good at detecting Hidden pages and will most likely respect the change, please do express caution with other search engines. If you are unsure, and the content is sensitive, it might be best to change the URL, or consider making the page 'Protected' for logged in members only.