Generally, above there is a server operating system applications that use the architecture of client / server. Examples of these applications is a DHCP Server, Mail Server, HTTP Server, FTP Server, DNS Server, and others. Each server operating systems generally bundle of services or the service can also be obtained from third parties. Each of these services will respond to requests from clients. For example, the DHCP client will give the request to the server that is running a DHCP server, when a client needs an IP address, the client will give the command / request to the server, with language that is understood by the DHCP server, the DHCP protocol itself.
Examples of server operating system is Windows NT 3.51, and continued with the Windows NT 4.0. Currently the system is quite popular is the Windows 2000 Server and Windows Server 2003, and Sun Solaris, Unix, and GNU / Linux.
Server is usually connected with a client with UTP cables and a Network Card. This network card is usually a PCI or ISA card.
Server function very much, for example for internet sites, science, or just data storage. But the most common is to connect the client computer to the Internet.