Apache

Definition by Web Hosting Ratings:

Apache is an open-source (source code is freely available and can be shared) HTTP Web server software.  According to Netcraft survey, it is currently the most popular web server on the Net.  It is usually run on Unix operating system versions like Linux or BSD, but it can also be run on Windows.  It is a full-featured server with many powerful add-ons freely available.  Apache's major competitor is Microsoft's IIS.