The Apache HTTP Server is a web server for Unix-like systems, Microsoft Windows, Novell NetWare and other operating systems. Apache is notable for playing a key role in the initial growth of the World Wide Web.
When first released, Apache was the only viable open source alternative to the Netscape Communications Corporation web server (currently known as Sun Java System Web Server). It has since evolved to rival other Unix-based web servers in terms of functionality and performance.
Since April 1996 Apache has been the most popular HTTP server on the World Wide Web, serving as the de facto reference platform against which other web servers are designed and judged; By May 1999 Apache installations served 57% of all websites. Its popularity continued to rise, and in February 2006 Apache served 68% of all websites. Apache continues to be the most popular web server in use.
Apache is developed and maintained by an open community of developers under the auspices of the Apache Software Foundation. Released under the Apache License, the Apache HTTP Server is free software.
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This video from Google Video shows you how easy it is to install an Apache server on a Windows XP operating system