The Apache HTTP Server, commonly referred to as Apache, is the most successful web server software. In 2009 it became the first web server software to surpass the 100 million web site milestone. Apache was the first viable alternative to the Netscape Communications Corporation web server (currently known as Sun Java System Web Server), and has since evolved to rival other Unix-based web servers in terms of functionality and performance. The majority of web servers using Apache run a Unix-like operating system like Linux.
Apache is developed and maintained by an open community of developers under the auspices of the Apache Software Foundation. The application is available for a wide variety of operating systems, including Unix, GNU, the BSDs, Linux, Solaris, Novell NetWare, Mac OS X, Microsoft Windows, and more. Released under the Apache License, Apache is characterized as open source software.
Since April 1996 Apache has been the most popular HTTP server software in use. As of February 2010 Apache served over 54.46% of all websites and over 66% of the million busiest.