Secure Shell, commonly known as SSH, and Telnet are network protocols that are widely used. They are both used to connect to remote servers in order to facilitate communication.
Key differentiators:
SSH and Telnet commonly serves the same purpose
SSH is more secure compared to Telnet
SSH encrypts the data while Telnet sends data in plain text
SSH uses a public key for authentication while Telnet does not use any authentication
SSH adds a bit more overhead to the bandwidth compared to Telnet
Telnet has been all but replaced by SSH in almost all uses