with paypal within the site user not to move paypal site user pay within the website and get his a/c registered as like that
there is any plugin that allow wordpress to get users paypal information. Plugins are available for wp and their work is to redirect to paypal site for payment.
There always a redirect to paypal website. I don't know if there's anyway to use paypal payment without actually going to paypal site.
if you dont want a paypal redirect and accept payments within your website you must have PayPal Pro account. Once you get paypalpro account only then you can accept payments within your website using paypal API. You will also need to have SSL enabled on your payment page to accept payments (i.e https://www.. not http://www... )