NordVPN is a service that uses the OpenVPN protocol. With NordVPN, you can connect through OpenVPN both over TCP and UDP. A protocol is a set of rules that determine how data is transmitted between different devices in the same network. It allows different devices to connect regardless of their inner proceses and design. A s ervice that uses a particular protocol uses this set of rules to connect to the server and give anonymity. This is what spells out the difference between NordVPN and OpenVPN.