Telnet is an application layer Internet protocol for remote access terminal emulation.
Telnet runs on the well-known TCP port 23. Telnet is character-oriented, transmitting one ASCII character per packet.
From a security standpoint, Telnet has many liabilities, most notably that all data and control messages are transmitted in clear text. The Telnet authentication mechanism also relies on a simple name-password combination, each character of which is also transmitted in the clear.
Telnet is defined through a long series of RFCs, beginning with RFC 15 in 1969. Hence, Telnet is one of the founding protocols of the Internet.