Apache HTTP Server is the world's most widely used Web server software. Apache is a freely available Web server that is distributed under an "open source" license. Version 2.0 runs on most UNIX-based operating systems such as Linux.
There is a great amount of modules created especially for the Apache server, which support various scripts and allow dynamic content to be run on the server. Most of the modules come as part of the Apache distribution, making the server boast a wide range of capabilities, such as support for CGI (Common Gateway Interface), a standard protocol for communication between external application software and a web server, and also SSI (Server Side Includes), a simple server-side scripting language. Other Apache modules include: CGI scripts execution, user authentication, URL redirection, anonymous user access, automatic directory listings, support for HTTP header metafiles, support for loading modules, content negotiation, caching proxy abilities, server status display, user home directories, etc. Users can choose to install those modules with the Apache server installation. If not, they can install them later with the help of dynamic modules.