In a technical sense, a server is an instance of a computer program or device that accepts and responds to requests made by another program, known as a client. A good metaphor would be a customer (client) ordering a package then the mailman (server) delivering it to them or someone else.
Servers are used to manage network resources. For example, a user may set up a server to control access to a network, send/receive e-mail, manage print jobs, or host a website. They are also proficient at performing intense calculations. Some servers are committed to a specific task, often referred to as dedicated. However, many servers today are shared servers which can take on the responsibility of e-mail, DNS, FTP, and even multiple websites in the case of a web server.
They are commonly used to deliver services that are constantly required, most servers are never turned off. Consequently, when servers fail, they can cause the network users and company many problems. To alleviate these issues, servers are commonly set up to be fault tolerant. This are type of servers used by mostly customer.