So far, telnet is been a very useful tool for hackers. It is a user command and an underlying TCP/IP protocol for accessing computers remotely.
The program runs on your pc and connects it to a server on the network and would be executed as if you were entering them directly on the server. This allows you to take charge of the server and communicate with other servers on the network.
Its default port is port 23.
For simplicity, you can logon to the system and use the files and stuff in it. It just requires that the intruder and the target are both connected to the internet. Every Windows computer has it. Users can access a host in any part of the world using this services. hackers use it to gain access to network devices.
A secured and better one is SSH which means Secure Socket Shell. It can prevent network sniffing and some hack attacks to an extent.
The idea behind using its server to connect to other systems is to be anonymous and look for open ports. Up to 5 or 6 ports are usually opened in a computer.
Know that, if a hacker can't connect to a computer, he won't be able to hack it.