1) In information technology, a server is a computer program that provides services to other computer programs (and their users) in the same or other computers.
2) The computer that a server program runs in is also frequently referred to as a server (though it may be used for other purposes as well).
3) In the client/server programming model, a server is a program that awaits and fulfills requests from client programs in the same or other computers. A given application in a computer may function as a client with requests for services from other programs and also as a server of requests from other programs.
Specific to the Web, a Web server is the computer program (housed in a computer) that serves requested HTML pages or files. A Web client is the requesting program associated with the user. The Web browser in your computer is a client that requests HTML files from Web servers.