What is telnet?

TELNET (TELecommunication NETwork)

it is a network protocol used
on the Internet or local area network (LAN) connections.
It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one
of the first Internet standards.
The term telnet also refers to software which implements
the client part of the protocol. TELNET clients have been available on
most Unix systems for many years and are available for virtually all platforms.
Most network equipment and OSs with a TCP/IP stack support some kind of TELNET
service server for their remote configuration (including ones based on Windows NT).
Recently, Secure Shell has begun to dominate remote access for Unix-based machines.
"To telnet" is also used as a
verb meaning to establish or use a TELNET or other interactive
TCP connection,as in, "To change your password, telnet to the server and run the passwd command".
Most often, a user will be telneting to a Unix-like server systemor a simple network device such as a switch.

For example, a user might "telnet in from home to check his mail at school".
In doing so, he would be using a telnet client to connect from his computer to one of his servers.
Once the connection is established, he would then log in with his account information and
execute operating system commands remotely on that computer, such as ls or cd.
On many systems, the client may also be used to make interactive raw-
TCP sessions,

even when that option is not available,
telnet sessions are equivalent to raw TCP as long as byte 255 never appears in the data.