Payflow Pro is a high performance TCP/IP-based internet payment solution that resides on your computer system.
Payflow Pro uses a client/server architecture to transfer transaction data from you to the processing networks, and
then returns the authorization results to you. Payflow Pro can process real-time credit card transactions and other
transaction types to most of the financial processing centers in the United States.
If you are implementing PayPal as a solution into your website, use this guide for general guidelines and guidelines
on integrating credit card processing.
Here is the link for Payflow Pro Developer's Guide: