A server is a running instance of an application capable of accepting requests from the client and giving responses accordingly.
A server computer (often just called server) is a computer system that has been designated for running a specific server application or applications. A server computer that is designated for generally only one server application is usually named for that application or purpose. For example, a server that runs Apache HTTP Server software, or Microsoft's IIS is typically designated as a web server, the computer running it is also simply called the web server. There can be web servers, database servers, application servers, such as: email servers or an Exchange Server, Share Point servers, Pre-press servers, Network Video Servers, Domain Controllers, and Network Storage or Backup Server. Server applications can be divided among application server computers over an extreme range, depending upon the workload.