PuTTY is a free implementation of Telnet and SSH for Win32 and Unix platforms, along with an xterm terminal emulator. SSH, Telnet and Rlogin are three ways of doing the same thing: logging in to a multi-user computer from another computer, over a network.
The Internet is a hostile environment and security is everybody's responsibility. If you are connecting across the open Internet, then we recommend you use SSH. If the server you want to connect to doesn't support SSH, it might be worth trying to persuade the administrator to install it.