Telnet is an application protocol used over any networks to insure bidirectional text-based communication facility using virtual terminal connection. The data is inserted in-band with Telnet control information in 8 bit byte orientated connection via TCP.
In simple words telnet allows you to connect to a remote computer from anywhere and use it as if you were sitting right in front of it. It was created in 1969, so as you might expect, security is woefully lacking. User discretion is advised.