In a general networking context, any device that responds to requests from client applications is functioning as a server. A server is usually a computer that contains information to be shared with many client systems. For example, web pages, documents, databases, pictures, video, and audio files can all be stored on a server and delivered to requesting clients. In other cases, such as a network printer, the print server delivers the client print requests to the specified printer.
Different types of server applications may have different requirements for client access. Some servers may require authentication of user account information to verify if the user has permission to access the requested data or to use a particular operation. Such servers rely on a central list of user accounts and the authorizations, or permissions, (both for data access and operations) granted to each user. When using an FTP client, for example, if you request to upload data to the FTP server, you may have permission to write to your individual folder but not to read other files on the site.
In a client/server network, the server runs a service, or process, sometimes called a server daemon. Like most services, daemons typically run in the background and are not under an end user's direct control. Daemons are described as "listening" for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon. When a daemon "hears" a request from a client, it exchanges appropriate messages with the client, as required by its protocol, and proceeds to send the requested data to the client in the proper format.