Sometimes, inexplicably it might seem, a program for your website may tell you that you need to run a command on the Web Host’s server. You may be asked to use SSH (Secure Shell) to accomplish this task. I was recently advised that I needed to perform a command on the server to let a WordPress Mobile Plug-in do its thing.
For that you will need a SSH client.
Putty is such an open source client. It is very much like a DOS window except that it won’t know DOS commands of course. For 1and1 hosting, the username and password is the same as for FTP.
There is no install program. You simply run the .exe from wherever you put it in your file system.
Putty is available at http://www.putty.org/. The download site is actually http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html. There is a user manual at http://the.earth.li/~sgtatham/putty/0.63/htmldoc/.