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Telnet is a user command using TCP/IP protocols to access a computer remotely. To have access to that computer, the user must have permission, meaning that user must authenticate to the system with a valid username and password. When you are connected to the network using telnet, you can enter commands and they will be executed as if they were being entered directly onto the server console.
Clients generally connect to port 23 on the host providing the service. Partly because of the design of the protocol and partly because of the flexibility typically provided by telnet programs, it is also possible to use a Telnet program to establish an interactive TCP connection to some other service on an Internet host. A typical use of this is telnetting to port 25 (where typically an SMTP server is to be found) to debug a mail server.
