I was wondering if ghost can also be extended like if you need a little of extra functionality besides just blogging. Like integrating payment with credit card, creating forms etc. or it's just about pages and posts ?
As for forms, I think if you wanted a completely custom form, then you would have to use something like TypeForm or JotForm and embed that into your Ghost blog.
It tries to focus solely on publishing and blogging, but recommends other services you can tie in if you need to which you can find here - https://ghost.org/integrations/