It usually relies on either Internet Protocol Security (IPsec) or Secure Sockets Layer (SSL) to secure the connection. However, SSL VPNs can also be used to supply secure access to a single application, rather than an entire internal network. Some VPNs also provide Layer 2 access to the target network; these will require a tunneling protocol like PPTP (Point-to-Point Tunneling Protocol) or L2TP (Layer 2 Tunneling Protocol) running across the base IPsec connection.
They even offer the most generous simultaneous connection count, with six simultaneous connections through their network, where everyone else offers five or fewer. NordVPN's network isn't as large as some of their competitors, so if you're trying to obfuscate your tracks, you might want a company with more servers. Otherwise, this company is clearly providing a winning offering.