Telnet is a basic Internet protocol that simulates the functionality of a character-by-character terminal, running over the Transmission Control Protocol. Even in a Web era, it remains important for accessing maintenance interfaces on network infrastructure.
Reverse telnet, in which a virtual window can speak to a number of serial interfaces through an appropriate application gateway, is a common means of accessing the consoles of multiple colocated routers or other network elements.
The well-known TCP port for Telnet is numbered 23. Most Telnet clients can go to alternate ports, and a Telnet server can redirect the session to a different address and port.
Since Internet application protocols are usually character-based, it can be used to test a variety of application protocols; "telnetting to Port 25", for example, allows Simple Mail Transfer Protocol messages to be constructed at a window and sent to the SMTP server.
Telnet is not built with security in mind - Telnet packets are not encrypted, and are thus easily 'sniffable'. For most user application purposes, it is generally recommended that people use Secure Shell (SSH) and its related protocols (SCP, SFTP etc.) which use public key cryptography and key-based authentication to solve the security problems in Telnet. There are purposes, however, where cryptographic security is not practical, such as the minimal-overhead command of an administrator interface. Another way of imposing security is to put the encryption into a reverse telnet server.
Do not confuse Telnet with TELENET, the name of an early commercial X.25 network.