OS (Operating System)
The Operating system is the basic controlling software loaded on the webserver Machine.
There are many operating systems for webservers i.e: Linux, Win2K, WinNT, Solaris, BSD various strains of Unix Etc..
The two most popular ones are Linux and Windows.
The only bearing it has on the website hosted is certain Microsoft technologies and softwares will not run on Linux Platform
What it means is if you are using ASP pages and MS-SQL server you will have to choose Windows, If you are using PHP pages and MySQL you will have to choose Linux.