A computer or device on a network that manages network resources. There are many different types of servers. For example
In the client/server model, a file server is a computer responsible for the central storage and management of data files so that other computers on the same network can access the files. A file server allows users to share information over a network without having to physically transfer files by floppy diskette or some other external storage device.
Manages the printers that are connected to the network and the printing of user documents on the network printers
Database server is the term used to refer to the back-end system of a database application using client/server architecture. The back-end, sometimes called a database server, performs tasks such as data analysis, storage, data manipulation, archiving, and other non-user specific tasks.
Web server is a computer where the web content is stored. Basically web server is used to host the web sites.
A mail server is a server that handles and delivers e-mail over a network, usually over the Internet. A mail server can receive e-mails from client computers and deliver them to other mail servers.
Domain controller is a server on a Microsoft Windows or Windows NT network that is responsible for allowing host access to Windows domain resources. The domain controllers in your network are the centerpiece of your Active Directory directory service. It stores user account information, authenticates users and enforces security policy for a Windows domain.