PuTTY is a free implementation of Telnet and SSH for Win32 and Unix platforms, along with an xterm terminal emulator. It is written and maintained primarily by Simon Tatham.
From their FAQ
A.1.1 What is PuTTY?
PuTTY is a client program for the SSH, Telnet and Rlogin network protocols.
These protocols are all used to run a remote session on a computer, over a network. PuTTY implements the client end of that session: the end at which the session is displayed, rather than the end at which it runs.
In really simple terms: you run PuTTY on a Windows machine, and tell it to connect to (for example) a Unix machine. PuTTY opens a window. Then, anything you type into that window is sent straight to the Unix machine, and everything the Unix machine sends back is displayed in the window. So you can work on the Unix machine as if you were sitting at its console, while actually sitting somewhere else.
I am not sure since when I have been using Putty. I think I have always used it. Though in between I had used ScureCRT and some more such similar terminals. But whatever be it I had to go back to one of the favorite clients on Windows for remote login - Putty. Well frankly I have not stopped experimenting other newer clients like Poderosa but they will have a long way to catch up with Putty. It is matured and is very well maintained, and is one of the most conforming to standards. Simon Tatham has made sure that what we have is one very light and fast pure Windows based client to match up to the UNIX/Linux version of the same. He with his team is working on porting it to Mac and others too. Any help according to him on porting from any quarter is welcome. So if you have the skills and are interested in similar development activities can help him out. Over the years I have found that colors and mouse support is excellent in Putty than compared to any other. Also the speed of communication seems to be visibly faster in Putty than many well known commercial clients. One of the features which I would like to see in Putty is tabbed terminals in a single window but somehow there seem to be some technical problems. As of now UNIX "screen", Wintabber and PuttyTabs help me through. For some more interesting tweaks and software based on Putty check out its Putty Links page
The PuTTY executables and source code are distributed under the MIT licence, which is similar in effect to the BSD licence. (This licence is Open Source certified and complies with the Debian Free Software Guidelines.)