rendered rerender renderas in salesforce explained
Rendered Attribute :-
The rendered attribute is a Boolean value, it has true as default value and based on the value it will show or hide a page block or a field on the VF pages.
rendered= "true or flase" Click here for rendered attribute example
Rerender Attribute :-
Rerender Attribute is used to refresh particular section on page to update latest data received from salesforce after action method call is done. You need to give the id of page section in attribute as shown below
rerender = "pageblockId" Click here for rerender attribute example used in actionfunction component
Renderas Attribute :-
If you want to render your page as PDF, so this is the right option to go ahead with it and most importantly only PDF is the supported content converter. Setting this Renderas attribute to “pdf” renders the page as a PDF.