Merchant Account: Service used to process credit card transactions.
Paypal is a merchant account and they have both a free version and a paid version. The free version requires you to have a Paypal account and when your customer checks out from your site, they will be directed to Paypal for the final payment screen.
Paypal Pro is Paypal's ‘professional' service that uses the same Paypal account, but your customers do not leave your site and go to Paypal to pay, they stay on your site.
Paypal Pro requires a monthly fee with Paypal regardless of if you process transactions or not and both the Free and Pro version take roughly the same fees per transaction.
If you would like to have your customers stay on your site, then we will likely need to help you set this up as it requires an SSL Cert also be installed (for the payment info going from your site to Paypal). There are several options that would help to know details about as well, if you would like to setup a call to talk through, please let us know so that we can get you the best solution for you and your customers needs.