From LQWiki

SSH is a secure replacement for Telnet as it allows you to encrypt all data traveling between computers. If you can, use ssh, not telnet!!!

Telnet is a protocol for terminal emulation across networks. The program used to connect to computers via this protocol is often known as telnet also.

Telnet allows you to execute commands on a remote system by typing them into your local console. Once ubiquitous for remote access usage, nowadays Telnet is discouraged for serious and secure work, since the protocol is insecure as all information is sent across a network in a plain text form.