PuTTY can use the Telnet protocol to connect to a server.
Telnet was perhaps the most popular remote login protocol before SSH was introduced. It was general enough to be used by multiple server operating systems (Unix and VMS in particular), and supported many optional protocol extensions providing extra support for particular server features.
Unlike SSH, Telnet runs over an unsecured network connection, so it is a very bad idea to use it over the hostile Internet (though it is still used to some extent as of 2020).