The Web Server (or HTTP Server) is the source of web pages returned to a Web Browser? when the server receives an HTTP Request from the browser. There are nearly 100 million web servers on the Internet, comprising the main engine of the World Wide Web?.

The original web server was built by Tim Berners Lee. The popular NCSA server, which was accompanied by the NCSA Mosaic browser, accounted for the rapid growth of the Web in the early nineties, but is now rarely seen.

The web is dominated by the Open Source Apache Web Server, accounting for about two-thirds of all servers and a still larger fraction of web pages served. Apache runs on all major platforms.

About 20% of web servers are Microsoft's Internet Information Server? (IIS). IIS runs only on the Windows platform.

References:
Wikipedia