1. A web is a computer on which a web site is hosted and a program that runs on such a computer. So the term web server refers to both hardware and software.
2. A web site is a collection of web pages generally written using Hyper Text Markup Language (HTML). For a web site to be available to everyone in the world at all times, it needs to be stored or “hosted” on a computer that is connected to the internet. Such a computer is known as a Web Server.
3. A web server program is software that runs on the web site hosting Server computer. Its main purpose is serving web pages for requests from web browsers.
4. The Server machine hosts (stores) the web site on its hard disk while the server program helps deliver the web pages and their associated files like images, flash movies, etc. to clients (browsers).
5. There are many web server programs available. The most famous and popular of them all is Apache developed by the Apache Foundation. It is free and also available for several operating systems including Windows, Macintosh, and Linux/Unix.