In a technical sense, a server is an instance of a computer program that accepts and responds to requests made by another program; known as a client. Less formally, any device that runs server software could be considered a server as well. Servers are used to manage network resources. For example, a user may setup a server to control access to a network, send/receive e-mail, manage print jobs, or host a website.
Some servers are committed to a specific task; often referred to as dedicated. As a result, there are a number of dedicated server categories, like print servers, file servers, network servers and database servers.
Because they are commonly used to deliver a services that are required constantly, most servers are never turned off. Consequently, when servers fail, they cause the network users or company many problems. To alleviate these issues, servers are commonly high-end computers setup to be fault tolerant.