PuTTY is a free PC application which allows you to connect to the webspace of your Linux hosting package via SSH to perform tasks or transfer data securely. Once connected via SSH, the PuTTY application displays a black and white text-based console much like the Windows Command Prompt.
From this command prompt you can Execute SSH Commands and perform some common tasks such as downloading files directly to your webspace, Changing Permissions for files in your webspace, managing Cron Jobs, etc.
While there may be other similar applications to make an SSH connection from a PC to a Linux server, PuTTY is the most well known and used. PuTTY is a freeware application which is maintained and updated occassionaly by its developers. You can find a link for the most current version from the Downloads section of the 1&1 Control Panel.