Double VPN - a privacy solution where your Internet traffic goes through two our servers and then reaches the Internet.
Onion over VPN - a privacy solution where your Internet traffic goes through one our servers then goes to the Onion network and then reaches the Internet (More Information about Onion over VPN).
Anti DDos - server suggested for less interrupting connection.
Dedicated IP servers - servers that have dedicated IPs.
P2P servers - servers compatible with Peer-to-Peer traffic. You can read more here: https://support.nordvpn.com/hc/en-us/articles/206939499-Peer-to-Peer-traffic-with-NordVPN
Standard server - a regular VPN server.