The Apache web server is based on the NCSA web server that was one of the most common UNIX-based web servers in the early and mid 90s. Originally it was a set of patches for NCSA httpd, hence the nickname "A patchy server".
Today it is one of the preeminent open source successes, in use on about half of the web servers on the Internet, and available for most UNIX variants and also Microsoft Windows as of Apache 1.3.
The Apache httpd distribution not only contains code for the httpd server daemon, but an assortment of utilties for managing certain server functions in user web pages, such as the htpasswd utility for creating and maintaining password-based access control files, an optional suexec setuid helper binary for executing user CGI programs, and an apachectl server management script, as well as extensive server documentation and an assortment of add-on modules for additional server functions. Apache versions since 2.0 also provide a portable runtime library and optional SSL support.
Versions of Apache before 2.0 maintained the NCSA httpd server model, where a master process managed a set of children which could independently handle requests; the master process could also fork more children to handle increased server load. Since 2.0 other server process models are available, taking advantage of lighter-weight "threads" in some UNIX versions (and Windows NT) to improve server performance.