PuTTY is a client program for the SSH, Telnet and Rlogin network protocols. For the purposes of web hosting, we mainly use it to operate the SSH protocol.
These protocols are all used to run a remote session on a computer over a network. PuTTY represents the clients’ side in those cases – you see where the session is displayed, rather than where it runs.
Many features and specific information can be found on the official PuTTY website.