Telnet is a TCP/IP based protocol which can be used to connect remote computer system through terminal emulation program. With telnet it is possible to connect any port if it is permitted on remote system. However in contrast to all applications that process data sending and receiving automatically, in telnet console user will have to provide commands manually for server applications running on remote system.
Most of the programs use TCP/IP protocol in transparent way which means that user is not informed about processes underneath user interface. With telnet it is possible to access the very same data flow in depth - to see in detail server responses and to send commands to server manually.
While nowadays the practical use of telnet is diminished by the role of various types of software which mediates data exchange with server, thus does not requiring from user to enter precisely formatted command line telnet strings, telnet might still come at hand for server accessibility testing and debugging purposes.
Note that telnet has serious security issues, e.g. data sent over telnet is non-encrypted, so that it can be easily stolen by data sniffing utilities. Avoid sending passwords or any other sensitive data over telnet. Opinions differ on what is telnet practical application nowadays, however if telnet is ever used in non-secure environments, it should be done with extreme caution.