A server is a device or computer on a network that's responsible for managing network resources. There are many examples of different functions of servers.  For example, a file server is a computer and storage device dedicated to storing files. Any user with access to the network can store files on the server.

A print server is a computer that manages one or more printers and a network server is a computer that's reponsible to manage network traffic. A database server is a computer system that processes database queries and so forth.

Servers are often dedicated, meaning that they perform no other tasks besides their server tasks. However, on multi-processing operating systems a single computer is able to execute several programs at the same time. A server in this case could refer to the application or program that's managing resources rather than the computer itself.