An operating system (OS) acts as a computer’s CEO — whether it operates on a server or a desktop computer. Just as a CEO controls all of the operations of a corporation, the OS controls all of a computer’s operations, such as allocating memory, accessing disk drives and calling applications, to name just a few. The software applications that provide the bulk of an OS’s services run on top of the OS. The three most popular operating systems for web servers are UNIX, Linux (a freeware version of UNIX), and some flavor of Windows (NT, 2000, 2003 Server).