The Operating System of the web server. This is usually some version of Linux.

You can develop your website on Windows and it will work on the Linux server just fine.

Rule of thumb: Most web servers are running Linux, for good reason. Unless you are an advanced programmer, you don't need to think about what OS the server is running.

If you are doing some advanced programming on Windows, especially if it is ASP or ASP.NET, then you will need a Windows server.