The Apache HTTP server project is an open source initiative for development and maintenance of the Apache Web server. The Apache Software Foundation oversees a geographically dispersed group of volunteer programmers collaborating on the server software development. Hundreds of users have also provided comments, suggestions, code and documentation to further the project.
"HTTP server" is another term for Web server. A Web server is a software program that maps URL requests from a Web client (typically a browser) to a resource that will handle the request and return a response to the client. The Web client and the Web server use HTTP to communicate over a TCP network.
Apache is the most commonly used Web server in the world. It runs on most operating systems, including Unix-based operating systems (such as Linux, Solaris, Digital UNIX, and AIX) and Unix-derived systems (such as BSD, FreeBSD and BeOS) Mac OS and Windows.
According to the project's website, the goal of the initiative is to create "a robust, commercial-grade, featureful, and freely-available source code implementation of an HTTP (Web) server."