Telnet is seldom used to connected to devices or systems anymore. Most devices, even very simple ones, can now be configured and managed via web based interfaces which are more secure and easy to use.
Telnet is also the name of a command available from the Command Prompt in Windows. The telnet command, unsurprisingly, is a command that uses the Telnet protocol to communicate with a device or system.
Note: You may also see Telnet referred to as an action like "I'll have to telnet into the switch to make those changes." The word telnet might also sometimes refer to whatever program is being used to communicate via the Telnet protocol.