Apache (HTTP Server) /əˈpætʃiː/ is a modular cross-platform web server software with a large variety of features(called modules) including SSL, compression, log, proxy and server-side programming language interfaces like Perl, Python, Tcl, and PHP.
The Apache HTTP Server, commonly referred to as Apache ( /əˈpætʃiː/), is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone.
Apache was the first viable alternative to the Netscape Communications Corporation web server (currently known as Oracle iPlanet Web Server), and has since evolved to rival other web servers in terms of functionality and performance. Typically Apache is run on a Unix-like operating system.
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, FreeBSD, linux, solaris, Novell NetWare, AmigaOS, Mac OS X, Microsoft windows, OS/2, TPF, and eComStation. Released under the Apache License, Apache is characterized as open-source software.
Apache supports a variety of features, many implemented as compiled modules which extend the core functionality. These can range from server-side programming language support to authentication schemes. Some common language interfaces support perl, python, tcl, and php. Popular authentication modules include mod_access, mod_auth, mod_digest, and mod_auth_digest, the successor to mod_digest. A sample of other features include SSL and TLS support (mod_ssl), a proxy module (mod_proxy), a URL rewriter (also known as a rewrite engine, implemented under mod_rewrite), custom log files (mod_log_config), and filtering support (mod_include and mod_ext_filter).
Popular compression methods on Apache include the external extension module, mod_gzip, implemented to help with reduction of the size (weight) of web pages served over HTTP. ModSecurity is an open source intrusion detection and prevention engine for web applications. Apache logs can be analyzed through a web browser using free scripts such as AWStats/W3Perl or Visitors.
Virtual Hosting allows one Apache installation to serve many different actual websites. For example, one machine with one Apache installation could simultaneously serve www.example.com, www.test.com, test47.test-server.test.com, etc.
Apache features configurable error messages, dbms-based authentication databases, and content negotiation. It is also supported by several graphical user interfaces (GUIs).
It supports password authentication and digital certificate authentication. Apache has a built in search engine and an html authorizing tool and supports ftp.
Although the main design goal of Apache is not to be the "fastest" web server, Apache does have performance comparable to other "high-performance" web servers. Instead of implementing a single architecture, Apache provides a variety of MultiProcessing Modules (MPMs) which allow Apache to run in a process-based, hybrid (process and thread) or event-hybrid mode, to better match the demands of each particular infrastructure. This implies that the choice of correct MPM and the correct configuration is important. Where compromises in performance need to be made, the design of Apache is to reduce latency and increase throughput, relative to simply handling more requests, thus ensuring consistent and reliable processing of requests within reasonable time-frames.
The Apache version considered by the Apache Foundation as providing high-performances is the multi-threaded version which mixes the use of several processes and several threads per process.
Unless you are asking about how to write Apache modules, questions with this tag probably belong on serverfault.com rather than stackoverflow.com.
