The TELNET (Teletype Network) protocol was developed in 1969 to communicate with a remote system via a CLI.
The command telnet uses the TELNET protocol for remote administration that relies on TCP. The default port used by telnet is 23.
telnet is not secure. It sends all the data, including usernames and passwords, in plaintext, making it easy to steal the login credentials. The secure alternative is SSH.
telnet, with its simplicity, can be used for other purposes. We can connect to any service running on TCP to grab its banner and even exchange a few messages unless it uses encryption.
Example
We want to discover more information about a web server at port 80.
We connect to the server at :80, and then we communicate using the HTTP protocol by issuing:
pentester@TryHackMe$ telnet MACHINE_IP 80
Trying MACHINE_IP...
Connected to MACHINE_IP.
Escape character is '^]'.
GET / HTTP/1.1
host: telnet
HTTP/1.1 200 OK
Server: nginx/1.6.2
Date: Tue, 17 Aug 2021 11:13:25 GMT
Content-Type: text/html
Content-Length: 867
Last-Modified: Tue, 17 Aug 2021 11:12:16 GMT
Connection: keep-alive
ETag: "611b9990-363"
Accept-Ranges: bytes
...Of particular interest for us is discovering the type and version of the installed web server, Server: nginx/1.6.2.