PuTTY is a free Telnet, SSH and xtherm terminal emulator for Windows and Unix platforms. Simon Tatham is mostly published and preserved.
These protocols are all used on a device over a network to run a remote session. PuTTY implements the termination of the session client: instead of the end at which the session is shown.
In very basic words, run PuTTY on a Windows computer and tell him (for example) to connect to a Unix computer. The programme opens a window. Then whatever you type is immediately sent to the Unix machine, and whatever Unix machine returns is seen in the browser. You will then function on the Unix computer as though you sat at the console when you were seated somewhere.