A protocol is the "language" chosen to communicate between two computers in networks.
Popular protocols include:
IP Internet Protocol
TCP Transmission Control
HTTP Hypertext Transfer
FTP File Transfer
Telnet Simple Remote Shell
SSH Secure Shell
UDP Universal Datagram (Raw)
POP3 Post Office (Mailbox)
SMTP Simple Mail Transfer (sendmail)
Jabber open source instant messages
IRC internet relay chat
and others ... (add here)